girepository: Fix leak in prefix_with_context

Found by Coverity.

https://bugzilla.redhat.com/show_bug.cgi?id=1938731

gitypelib.c

@@ -2139,7 +2139,7 @@ prefix_with_context (GError **error,
 		     const char *section,
 		     ValidateContext *ctx)
 {
-  GString *str = g_string_new (NULL);
+  GString *str;
   GSList *link;
   char *buf;
 
@@ -2150,6 +2150,8 @@ prefix_with_context (GError **error,
       return;
     }
 
+  str = g_string_new (NULL);
+
   for (; link; link = link->next)
     {
       g_string_append (str, link->data);