Read the record length in common code.

Instead of reading the 16-byte blob and record length at the same time,
just read the 16-byte blob, and then fall through to the record-length
reading code.

Change-Id: Ib2819a2d654e2670233821882bac79d7cd656b12
Reviewed-on: https://code.wireshark.org/review/4480
Reviewed-by: Guy Harris <guy@alum.mit.edu>