avformat/img2dec: Little JFIF / Exif cleanup

This changes the behavior and makes it behave how it probably was intended.
Either way this is unlikely to result in any user visible change

Fixes: CID1494637 Missing break in switch

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5712f36dd0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/img2dec.c

@@ -793,11 +793,13 @@ static int jpeg_probe(const AVProbeData *p)
             state = EOI;
             break;
         case APP0:
-            if (AV_RL32(&b[i + 4]) == MKTAG('J','F','I','F'))
+            if (c == APP0 && AV_RL32(&b[i + 4]) == MKTAG('J','F','I','F'))
                 got_header = 1;
+            /* fallthrough */
         case APP1:
-            if (AV_RL32(&b[i + 4]) == MKTAG('E','x','i','f'))
+            if (c == APP1 && AV_RL32(&b[i + 4]) == MKTAG('E','x','i','f'))
                 got_header = 1;
+            /* fallthrough */
         case APP2:
         case APP3:
         case APP4: