Darwin/ARM: Assert on NULL dereference

This inhibits an analyzer warning by Clang on all platforms.

src/arm/ffi.c

@@ -350,11 +350,19 @@ void ffi_call(ffi_cif *cif, void (*fn)(void), void *rvalue, void **avalue)
       break;
     }
   if (small_struct)
+    {
+      FFI_ASSERT(rvalue != NULL);
       memcpy (rvalue, &temp, cif->rtype->size);
+    }
+    
   else if (vfp_struct)
+    {
+      FFI_ASSERT(rvalue != NULL);
       memcpy (rvalue, ecif.rvalue, cif->rtype->size);
     }
     
+}
+
 /** private members **/
 
 static void ffi_prep_incoming_args_SYSV (char *stack, void **ret,