avcodec/mjpegdec: Clip DC also on the negative side.

Fixes: runtime error: signed integer overflow: -16711425 + -2130772346 cannot be represented in type 'int'
Fixes: 2533/clusterfuzz-testcase-minimized-5372857678823424

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c28f648b19)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegdec.c

@@ -687,7 +687,7 @@ static int decode_block(MJpegDecodeContext *s, int16_t *block, int component,
         return AVERROR_INVALIDDATA;
     }
     val = val * quant_matrix[0] + s->last_dc[component];
-    val = FFMIN(val, 32767);
+    val = av_clip_int16(val);
     s->last_dc[component] = val;
     block[0] = val;
     /* AC coefs */