jpeg: Check for integer overflows in app1 EXIF tags

In jpeg_parse_exif_app1(), we would usually read offsets this way:

    /* read out the offset pointer to IFD0 */
    offset  = de_get32(&marker->data[i] + 4, endian);
    i = i + offset;

"i" is then used to peek into the buffer and read bytes.

	tags = de_get16(&marker->data[i], endian);
	i = i + 2;

But as the addition may overflow, we need to check whether the result of
the addition would overflow and wrap-around.

https://bugzilla.gnome.org/show_bug.cgi?id=775218