Ensure that the path is always set

We are using `path` unconditionally, but it can be conditionally filled.
To avoid inconsistent internal state, and a compiler warning, let's
assert that the variable is always set.

gdk/gdkcontentserializer.c

@@ -615,7 +615,7 @@ static void
 file_text_serializer (GdkContentSerializer *serializer)
 {
   const GValue *value;
-  char *path;
+  char *path = NULL;
 
   value = gdk_content_serializer_get_value (serializer);
 
@@ -651,6 +651,8 @@ file_text_serializer (GdkContentSerializer *serializer)
       path = g_string_free (str, FALSE);
     }
 
+  g_assert (path != NULL);
+
   g_output_stream_write_all_async (gdk_content_serializer_get_output_stream (serializer),
                                    path,
                                    strlen (path),