Fix a bad return value in ws_addarg()

Fix a bad return value in ws_addarg() when updating a header value,
leading the caller to believe an out of memory condition happened.

src/webserver.c

@@ -1516,7 +1516,7 @@ int ws_addarg(ARGLIST *root, char *key, char *fmt, ...) {
             free(newkey);
             free(pnew);
             WS_EXIT();
-            return 0;
+            return TRUE;
         }
         current=current->next;
     }