Remove misuse of wtap_file_read_expected_bytes().

wtap_file_read_expected_bytes() is a macro that can return a Boolean
FALSE; it should not be used in routines that don't return a Boolean.

In addition, both EOF *and* a short read, in that routine, should be
treated as a "not an IPFIX file" indication.

While we're at it, a seek failure should be treated as an error.

Change-Id: I97815bc9e78169ded567b60835cc7bcf6a0e6f0c
Reviewed-on: https://code.wireshark.org/review/4261
Reviewed-by: Guy Harris <guy@alum.mit.edu>