stb_image: Tweak end-of-JPEG junk scanning loop.

Be a bit more picky about what we consider a valid marker.
0xff 0x00 (stuffed 0 byte) should not count.

Fixes issue #1409.

stb_image.h

@@ -3383,6 +3383,28 @@ static int stbi__decode_jpeg_header(stbi__jpeg *z, int scan)
    return 1;
 }
 
+static int stbi__skip_jpeg_junk_at_end(stbi__jpeg *j)
+{
+   // some JPEGs have junk at end, skip over it but if we find what looks
+   // like a valid marker, resume there
+   while (!stbi__at_eof(j->s)) {
+      int x = stbi__get8(j->s);
+      while (x == 255) { // might be a marker
+         if (stbi__at_eof(j->s)) return STBI__MARKER_none;
+         x = stbi__get8(j->s);
+         if (x != 0x00 && x != 0xff) {
+            // not a stuffed zero or lead-in to another marker, looks
+            // like an actual marker, return it
+            return x;
+         }
+         // stuffed zero has x=0 now which ends the loop, meaning we go
+         // back to regular scan loop.
+         // repeated 0xff keeps trying to read the next byte of the marker.
+      }
+   }
+   return STBI__MARKER_none;
+}
+
 // decode image to YCbCr format
 static int stbi__decode_jpeg_image(stbi__jpeg *j)
 {
@@ -3399,14 +3421,7 @@ static int stbi__decode_jpeg_image(stbi__jpeg *j)
          if (!stbi__process_scan_header(j)) return 0;
          if (!stbi__parse_entropy_coded_data(j)) return 0;
          if (j->marker == STBI__MARKER_none ) {
-            // handle 0s at the end of image data from IP Kamera 9060
-            while (!stbi__at_eof(j->s)) {
-               int x = stbi__get8(j->s);
-               if (x == 255) {
-                  j->marker = stbi__get8(j->s);
-                  break;
-               }
-            }
+         j->marker = stbi__skip_jpeg_junk_at_end(j);
             // if we reach eof without hitting a marker, stbi__get_marker() below will fail and we'll eventually return 0
          }
       } else if (stbi__DNL(m)) {