Don't assume pcap_read_block() returns the number of bytes read.

Recent changes must have made it not do so in some cases, and
our current version of file_tell(), unlike ftell() around which the
older file_tell() was a wrapper, is known to be cheap (ftell() would
make a system call to get the position).  Just use file_tell() before
each read.

(Further cleanup is called for.)

Bug: 10568
Change-Id: Ib92057b2b87ec6eb16fd612bc91baeb668d1e1c7
Reviewed-on: https://code.wireshark.org/review/4691
Reviewed-by: Guy Harris <guy@alum.mit.edu>