avcodec/dfa: Fix signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'

Fixes: 1368/clusterfuzz-testcase-minimized-4507293276176384

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 12936a4585)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dfa.c

@@ -67,7 +67,8 @@ static int decode_tsw1(GetByteContext *gb, uint8_t *frame, int width, int height
     const uint8_t *frame_start = frame;
     const uint8_t *frame_end   = frame + width * height;
     int mask = 0x10000, bitbuf = 0;
-    int v, count, segments;
+    int v, count;
+    unsigned segments;
     unsigned offset;
 
     segments = bytestream2_get_le32(gb);