avcodec/h264_parse: Use 64bit for expectedpoc and expected_delta_per_poc_cycle

Fixes: signed integer overflow: -2142516591 + -267814575 cannot be represented in type 'int'
Fixes: 14450/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5716105319940096

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_parse.c

@@ -301,7 +301,8 @@ int ff_h264_init_poc(int pic_field_poc[2], int *pic_poc,
         if (picture_structure == PICT_FRAME)
             field_poc[1] += pc->delta_poc_bottom;
     } else if (sps->poc_type == 1) {
-        int abs_frame_num, expected_delta_per_poc_cycle, expectedpoc;
+        int abs_frame_num;
+        int64_t expected_delta_per_poc_cycle, expectedpoc;
         int i;
 
         if (sps->poc_cycle_length != 0)