repacketizer_demo: check for read errors to fix compiler warnings

Actually check for read errors instead of just storing the
return value in a variable that then never gets checked.

Also fixes "conversion from 'size_t' to 'int', possible loss
of data" compiler warnings on Windows with MSVC caused by
storing the size_t returned by fread() into an int variable.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>