Free file header in virStorageFileGetMetadataRecurse

Introduced by commit 2bdb8b96

src/storage/storage_driver.c

@@ -2906,6 +2906,7 @@ virStorageFileGetMetadataRecurse(virStorageSourcePtr src,
     ret = 0;
 
  cleanup:
+    VIR_FREE(buf);
     virStorageFileDeinit(src);
     virStorageSourceFree(backingStore);
     return ret;