virDomainDiskSourceFormatInternal: Avoid leaking @childBuf
If formatting of storage encryption or private data fails we must
jump to the error label instead of returning immediately
otherwise @attrBuf and @childBuf might be leaked.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>