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>

libavcodec/mpeg4videoenc.c

@@ -882,7 +882,7 @@ void ff_set_mpeg4_time(MpegEncContext *s)
 
 static void mpeg4_encode_gop_header(MpegEncContext *s)
 {
-    int hours, minutes, seconds;
+    int64_t hours, minutes, seconds;
     int64_t time;
 
     put_bits(&s->pb, 16, 0);