Tock OS fault with IPC and Nordic ble-uart

fault.txt

Hi! We're having quite some trouble getting an IPC client app to work together with BLE on hail.
We're using the `ble-uart` example app and just included these lines:

```
static char buf[64] __attribute__((aligned(64)));
int hotp_svc_num = 0;
bool calledback = false;
static void hotp_callback(__attribute__ ((unused)) int pid,
                           __attribute__ ((unused)) int len,
                           __attribute__ ((unused)) int arg2, void* ud) {
  calledback = true;
}
```

and in the main function before the example code

```
hotp_svc_num = ipc_discover("hotpservice"); // We've checked, this does not return an error
ipc_register_client_cb(hotp_svc_num, hotp_callback, buf);
```

We've been trying to debug this for a long time now but don't really know what to do anymore.
Removing the `ipc_register_client_cb`-call from main works, but with it, the following fault happens:

```
Kernel panic at /.../tock/kernel/src/process.rs:537:                                                               
        "Process ble-uart had a fault"                                                                                                
        Kernel version release-1.2-2018-06-985-gc4ffb50b                                                                              
                                                                                                                                      
---| Debug buffer not empty. Flushing. May repeat some of last message(s):                                                            
Initialization complete. Entering main loop                                                                                           
                                                                                                                                      
---| Fault Status |---                                                                                                                
Data Access Violation:              true                           
Forced Hard Fault:                  true                           
Faulting Memory Address:            0x20005000                                                                                        
Fault Status Register (CFSR):       0x00000082               
Hard Fault Status Register (HFSR):  0x40000000                     
                                                                   
---| App Status |---                                               
App: ble-uart   -   [Fault]                                                                                                           
 Events Queued: 0   Syscall Count: 3   Dropped Callback Count: 0
 Restart Count: 0                                                  
 Last Syscall: Some(MEMOP { operand: 11, arg0: 536896392 })
                                                                   
 ╔═══════════╤══════════════════════════════════════════╗
 ║  Address  │ Region Name    Used | Allocated (bytes)  ║
 ╚0x20006000═╪══════════════════════════════════════════╝
             │ ▼ Grant         800 |    800          
  0x20005CE0 ┼───────────────────────────────────────────
             │ Unused
  0x20004C00 ┼───────────────────────────────────────────
             │ ▲ Heap       536890368 | 536894688               S
  0x00000000 ┼─────────────────────────────────────────── R
             │ Data         3758077952 | 3758077952               A 
  0x20004800 ┼─────────────────────────────────────────── M
             │ ▼ Stack          40 |   2048          
  0x200047D8 ┼───────────────────────────────────────────
             │ Unused
  0x20004000 ┴───────────────────────────────────────────
             .....
  0x00040000 ┬─────────────────────────────────────────── F
             │ App Flash     65492                        L
  0x0003002C ┼─────────────────────────────────────────── A
             │ Protected        44                        S
  0x00030000 ┴─────────────────────────────────────────── H

  R0 : 0x20004BC0    R6 : 0x0003002C
  R1 : 0x00000000    R7 : 0x20004000
  R2 : 0x20006388    R8 : 0x00000000
  R3 : 0x20005000    R10: 0x00000000
  R4 : 0x0003002C    R11: 0x00000000
  R5 : 0x20004800    R12: 0x00000000
  R9 : 0x20004800 (Static Base Register)
  SP : 0x200047D8 (Process Stack Pointer)
  LR : 0x000306E7
  PC : 0x00036BB0
 YPC : 0x00000000

 APSR: N 1 Z 0 C 0 V 0 Q 0
       GE 0 0 0 0
 EPSR: ICI.IT 0x00
       ThumbBit true 
To debug, run `make debug RAM_START=0x20004000 FLASH_INIT=0x30055`
in the app's folder and open the .lst file.

App: hotpservice   -   [Yielded]
[...]
```

We'd be glad for any hints.