Protect against a 0-length for parameters as well.

Summary:
Encountered with a tag of 0 (attachmentname), but that might be
coincidence. We are already protecting above against 0-length which we
may run into if the stream is truncated for some reason.

Since this is not treated above as an error, I'm not treating it as one
here either.

Differential Revision: https://git.kolab.org/D1468