avcodec/mjpeg_parser: Adjust size rejection threshold

Fixes: 86987846-429c8d80-c197-11ea-916b-bb4738e09687.jpg
Fixes: Regression since ec3d8a0e69

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit dde6077297)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpeg_parser.c

@@ -82,7 +82,7 @@ static int find_frame_end(MJPEGParserContext *m, const uint8_t *buf, int buf_siz
                     return i-3;
                 } else if(state<0xFFD00000 || state>0xFFD9FFFF){
                     m->size= (state&0xFFFF)-1;
-                    if (m->size >= 0x8000)
+                    if (m->size >= 0xF000)
                         m->size = 0;
                 }
             }