av_find_stream_info: change read_size to 64bit

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat: Fix infinite probing that reads the whole file

Fixes: otonajoshi_avformat_tries_to_load_whole.ts
Reported-by: JEEB (on IRC)
Thanks-to: nevcairiel
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>