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>
diff --git a/shared/config-parser.c b/shared/config-parser.c
index e1bf212..8defbbb 100644
--- a/shared/config-parser.c
+++ b/shared/config-parser.c
@@ -385,7 +385,7 @@
 const char *
 weston_config_get_full_path(struct weston_config *config)
 {
-	return config->path;
+	return config == NULL ? NULL : config->path;
 }
 
 int