SEGV when MVM_DEBUG_NFG 1 and MVM_DEBUG_NFG_STRICT 0

gistfile1.txt

dogbert@dogbert-VirtualBox ~/repos/rakudo $ ./perl6-gdb-m t/02-rakudo/08-repeat.t
================================================================================================
This is Rakudo Perl 6 running in the GNU debugger, which often allows the user to generate useful back-
traces to debug or report issues in Rakudo, the MoarVM backend or the currently running code.

This Rakudo version is 2017.10.201.g.0.b.35.c.4.adc built on MoarVM version 2017.10.80.ga.0.dc.2747,
running on ubuntu (14.04.3.LTS.Trusty.Tahr) / linux (3.19.0.32.generic)

Type `bt full` to generate a backtrace if applicable, type `q` to quit or `help` for help.
------------------------------------------------------------------------------------------------
Reading symbols from /home/dogbert/repos/rakudo/nqp/MoarVM/../../install/bin/moar...done.
Starting program: /home/dogbert/repos/rakudo/install/bin/moar --execname=./perl6-gdb-m --libpath=. --libpath=blib --libpath=/home/dogbert/repos/rakudo/install/share/nqp/lib --libpath=/home/dogbert/repos/rakudo/install/share/nqp/lib --libpath=/home/dogbert/repos/rakudo/install/share/nqp/lib /home/dogbert/repos/rakudo/perl6.moarvm --nqp-lib=blib t/02-rakudo/08-repeat.t
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
[New Thread 0xb7449b40 (LWP 17567)]
1..5

Program received signal SIGSEGV, Segmentation fault.
0xb7c59a97 in re_nfg (tc=0x804c5f8, in=0xb7886598) at src/strings/ops.c:287
287	            out_buffer[out_pos++] = g;
(gdb) bt
#0  0xb7c59a97 in re_nfg (tc=0x804c5f8, in=0xb7886598) at src/strings/ops.c:287
#1  0xb7c58f23 in NFG_checker (tc=0x804c5f8, orig=0xb7886598, varname=0xb7cc2541 "string ‘a’") at src/strings/ops.c:80
#2  0xb7c5915f in NFG_check_concat (tc=0x804c5f8, result=0xb7886838, a=0xb7886598, b=0xb78865f0, varname=0xb7cc2811 "'result'") at src/strings/ops.c:120
#3  0xb7c5b8ff in MVM_string_concatenate (tc=0x804c5f8, a=0xb7886598, b=0xb78865f0) at src/strings/ops.c:721
#4  0xb7b7d453 in MVM_interp_run (tc=0x804c5f8, initial_invoke=0xb7c82f27 <toplevel_initial_invoke>, invoke_data=0x80b0500) at src/core/interp.c:1572
#5  0xb7c83086 in MVM_vm_run_file (instance=0x804c008, filename=0xbffff2e1 "/home/dogbert/repos/rakudo/perl6.moarvm") at src/moar.c:401
#6  0x08048fe6 in main (argc=10, argv=0xbfffeff4) at src/main.c:256