Merge pull request #1303 from Unity-Technologies/stackwalking-handle-null-ip

Gather callstack even if instruction pointer is null.

Author: joncham

mono/mini/mini-exceptions.c

@@ -2727,7 +2727,7 @@ mono_unity_backtrace_from_context (void* context, void* array[], int count)
 	ip = (void*)MONO_CONTEXT_GET_IP(&mctx);
 	bp = (void**)MONO_CONTEXT_GET_BP(&mctx);
 
-	while(ip && bp && count-- > 0)
+	while(bp && count-- > 0)
 	{
 		array[idx++] = ip;