jvdec: avoid unsigned overflow in comparison

The return type of strlen is size_t, i.e. unsigned, so if pd->buf_size
is 3, the right side overflows leading to a wrong result of the
comparison and subsequently a heap buffer overflow.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/jvdec.c

@@ -54,7 +54,7 @@ typedef struct JVDemuxContext {
 
 static int read_probe(AVProbeData *pd)
 {
-    if (pd->buf[0] == 'J' && pd->buf[1] == 'V' && strlen(MAGIC) <= pd->buf_size - 4 &&
+    if (pd->buf[0] == 'J' && pd->buf[1] == 'V' && strlen(MAGIC) + 4 <= pd->buf_size &&
         !memcmp(pd->buf + 4, MAGIC, strlen(MAGIC)))
         return AVPROBE_SCORE_MAX;
     return 0;