Set stack_start of rb_thread_t in ruby_init_stack()

thread_pthread.patch

diff --git a/thread_pthread.c b/thread_pthread.c
index eb98024..30ca276 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -45,6 +45,7 @@ static void native_cond_wait(rb_thread_cond_t *cond, pthread_mutex_t *mutex);
 static void native_cond_initialize(rb_thread_cond_t *cond, int flags);
 static void native_cond_destroy(rb_thread_cond_t *cond);
 static void rb_thread_wakeup_timer_thread_low(void);
+static int native_thread_init_stack(rb_thread_t *th);
 static pthread_t timer_thread_id;
 
 #define RB_CONDATTR_CLOCK_MONOTONIC 1
@@ -690,6 +691,12 @@ ruby_init_stack(volatile VALUE *addr
 	    native_main_thread.stack_maxsize = 0; /* unknown */
 	}
     }
+    {
+        rb_thread_t *th = GET_THREAD();
+        if (th && pthread_equal(th->thread_id, native_main_thread.id)) {
+            native_thread_init_stack(th);
+        }
+    }
 }
 
 #define CHECK_ERR(expr) \