libavcodec/mjpeg: preserve unclipped last_dc value

Perform av_clip_int16(val) _after_ copying the value to last_dc.

This change ensures that clipping is applied only within the context of
the current block, preventing the propagation of clipped values to
subsequent DC components.

Related commits: c28f648b19 and dffae122d0
Related ticket: 4683

libavcodec/mjpegdec.c

@@ -843,9 +843,8 @@ static int decode_block(MJpegDecodeContext *s, int16_t *block, int component,
         return AVERROR_INVALIDDATA;
     }
     val = val * (unsigned)quant_matrix[0] + s->last_dc[component];
-    val = av_clip_int16(val);
     s->last_dc[component] = val;
-    block[0] = val;
+    block[0] = av_clip_int16(val);
     /* AC coefs */
     i = 0;
     {OPEN_READER(re, &s->gb);

tests/ref/fate/jpg-12bpp

@@ -3,4 +3,4 @@
 #codec_id 0: rawvideo
 #dimensions 0: 999x749
 #sar 0: 1/1
-0,          0,          0,        1,  1496502, 0xd91deb4b
+0,          0,          0,        1,  1496502, 0x44efc0af