Fix a record size check.

It was being checked against the wrong value, so some invalid records
passed the check.

Also, change one comparison (rec_size is in the range [0, 65535], even
though it's in an int, so we can safely cast it to guint) and fix the
metadata length value when reading Ethernet packets.

Bug: 10495
Change-Id: I2ce5c93fe50d836ec0accfcdef31654ba6b5b7c7
Reviewed-on: https://code.wireshark.org/review/4278
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Reviewed-on: https://code.wireshark.org/review/4279