Squelch some used-but-not-set warnings (one legitimate, others not).

Also, we don't care how much tagged value data we've read, we just care
whether we've read any or not (EOF reading the first one means "end of
file", EOF reading any of the subsequent ones means "short read").

Change-Id: I2edc54494967b3a88bcc2c79d97eedfded00150d
Reviewed-on: https://code.wireshark.org/review/4993
Reviewed-by: Guy Harris <guy@alum.mit.edu>