lavf/img2dec: Improve jpeg auto-detection.

Fixes ticket #4841.

libavformat/img2dec.c

@@ -660,18 +660,14 @@ static int j2k_probe(AVProbeData *p)
 static int jpeg_probe(AVProbeData *p)
 {
     const uint8_t *b = p->buf;
-    int i, state = 0xD8, exif_size = 0;
+    int i, state = 0xD8;
 
     if (AV_RB16(b) != 0xFFD8 ||
         AV_RB32(b) == 0xFFD8FFF7)
     return 0;
 
     b += 2;
-    if (AV_RB16(b) == 0xFFE1 && AV_RB32(b + 4) == AV_RB32("Exif")) {
-        exif_size = AV_RB16(b + 2) + 2;
-        b += exif_size;
-    }
-    for (i = 0; i + exif_size < p->buf_size - 2; i++) {
+    for (i = 0; i < p->buf_size - 3; i++) {
         int c;
         if (b[i] != 0xFF)
             continue;
@@ -700,6 +696,24 @@ static int jpeg_probe(AVProbeData *p)
                 return 0;
             state = 0xD9;
             break;
+        case 0xE0:
+        case 0xE1:
+        case 0xE2:
+        case 0xE3:
+        case 0xE4:
+        case 0xE5:
+        case 0xE6:
+        case 0xE7:
+        case 0xE8:
+        case 0xE9:
+        case 0xEA:
+        case 0xEB:
+        case 0xEC:
+        case 0xED:
+        case 0xEE:
+        case 0xEF:
+            i += AV_RB16(&b[i + 2]) + 1;
+            break;
         default:
             if (  (c >= 0x02 && c <= 0xBF)
                 || c == 0xC8)