jpegparse: return offset+length from _get_image_length().

_find_end_marker() find the position of the marker. EOI has a fixed length of 2
bytes that where missing. Fixes #608998

gst/jpegformat/gstjpegparse.c

@@ -425,8 +425,8 @@ gst_jpeg_parse_get_image_length (GstJpegParse * parse)
       offset = gst_jpeg_parse_find_end_marker (parse, data, size);
     }
   }
-
-  return offset;
+  /* position of EOI + the length of the marker */
+  return offset + 2;
 }
 
 static gboolean