Add error-checking for the case where _arg_cache_new() fails

This can happen when a typelib and its underlying library are out-of-sync. This
converts the segfault into a more helpful traceback.

https://bugzilla.gnome.org/show_bug.cgi?id=678914

gi/pygi-cache.c

@@ -1277,6 +1277,8 @@ _args_cache_generate (GICallableInfo *callable_info,
                         return_direction,
                         -1,
                         -1);
+    if (return_cache == NULL)
+        return FALSE;
 
     return_cache->is_skipped = g_callable_info_skip_return (callable_info);
     callable_cache->return_cache = return_cache;