config: Don't crash if we don't have a config file

Adding a check in weston_config_full_path so that
we don't crash if we started without a config file.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>

shared/config-parser.c

@@ -385,7 +385,7 @@ weston_config_parse(const char *name)
 const char *
 weston_config_get_full_path(struct weston_config *config)
 {
-	return config->path;
+	return config == NULL ? NULL : config->path;
 }
 
 int