avcodec/mpeg4videoenc: Use 64 bit for times in mpeg4_encode_gop_header()

      Fixes truncation
Fixes Assertion n <= 31 && value < (1U << n) failed at libavcodec/put_bits.h:169
Fixes: ffmpeg_crash_2.avi

Found-by: Thuan Pham <thuanpv@comp.nus.edu.sg>, Marcel Böhme, Andrew Santosa and Alexandru RazvanCaciulescu with AFLSmart

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>