radcom_read_rec() should always read from the file specified by the fh argument.

It's called both from the read and seek-read routines, so it shouldn't
always read from the sequential handle.

Change-Id: I8cb33b9f5b7219f335b0aeeef29c479916276f89
Reviewed-on: https://code.wireshark.org/review/4276
Reviewed-by: Guy Harris <guy@alum.mit.edu>