Make peekclassic_read_packet_v56() more like the _v7 version.

We can't use wtap_file_read_expected_bytes() in the _v7 version, as that
version returns an int, not a Boolean; just expand
wtap_file_read_expected_bytes() in the _v56 version, to make it look
similar to the _v7 version.

Change-Id: Id907bac265c123ad5821591c1cf081b5747724d8
Reviewed-on: https://code.wireshark.org/review/4262
Reviewed-by: Guy Harris <guy@alum.mit.edu>