avcodec/cbs_h264: fix storage type for time_offset in Pic Timing SEI

The spec defines it as a signed value.
Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/cbs_h264.h

@@ -253,7 +253,7 @@ typedef struct H264RawSEIPicTimestamp {
     uint8_t minutes_value;
     uint8_t hours_flag;
     uint8_t hours_value;
-    uint32_t time_offset;
+    int32_t time_offset;
 } H264RawSEIPicTimestamp;
 
 typedef struct H264RawSEIPicTiming {

libavcodec/cbs_h264_syntax_template.c

@@ -592,8 +592,9 @@ static int FUNC(sei_pic_timestamp)(CodedBitstreamContext *ctx, RWContext *rw,
         time_offset_length = 24;
 
     if (time_offset_length > 0)
-        u(time_offset_length, time_offset,
-          0, MAX_UINT_BITS(time_offset_length));
+        i(time_offset_length, time_offset,
+          MIN_INT_BITS(time_offset_length),
+          MAX_INT_BITS(time_offset_length));
     else
         infer(time_offset, 0);