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

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.