smvjpegdec: make sure cur_frame is not negative

This fixes a heap-buffer-overflow detected by AddressSanitizer.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/smvjpegdec.c

@@ -152,6 +152,10 @@ static int smvjpeg_decode_frame(AVCodecContext *avctx, void *data, int *data_siz
 
     cur_frame = avpkt->pts % s->frames_per_jpeg;
 
+    /* cur_frame is later used to calculate the buffer offset, so it mustn't be negative */
+    if (cur_frame < 0)
+        cur_frame += s->frames_per_jpeg;
+
     /* Are we at the start of a block? */
     if (!cur_frame) {
         av_frame_unref(mjpeg_data);