avformat/mpjpegdec: fix finding multipart boundary parameter

The string matching function's return value was evaluated incorrectly.

Fixes trac #7920.
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpjpegdec.c

@@ -267,7 +267,7 @@ static char* mpjpeg_get_boundary(AVIOContext* pb)
         while (av_isspace(*start))
             start++;
 
-        if (!av_stristart(start, "boundary=", &start)) {
+        if (av_stristart(start, "boundary=", &start)) {
             end = strchr(start, ';');
             if (end)
                 len = end - start - 1;