storage_backend_fs: Don't free a part of a structure on error

As the storage pool sources are stored in a list of structs, the pointer
returned by virStoragePoolSourceListNewSource() shouldn't be freed as it
points in the middle of a memory block. This combined with a regression
that takes the error path every time on caused a double-free abort on
the src struct in question.
(cherry picked from commit ab9c72ae9e)

src/storage/storage_backend_fs.c

@@ -213,10 +213,8 @@ virStorageBackendFileSystemNetFindPoolSourcesFunc(virStoragePoolObjPtr pool ATTR
     }
     src->format = VIR_STORAGE_POOL_NETFS_NFS;
 
-    src = NULL;
     ret = 0;
 cleanup:
-    virStoragePoolSourceFree(src);
     return ret;
 }