Improve deep recursion detection in exif_data_load_data_content.

The existing detection was still vulnerable to pathological cases
causing DoS by wasting CPU. The new algorithm takes the number of tags
into account to make it harder to abuse by cases using shallow recursion
but with a very large number of tags.  This improves on commit 5d28011c
which wasn't sufficient to counter this kind of case.

The limitation in the previous fix was discovered by Laurent Delosieres,
Secunia Research at Flexera (Secunia Advisory SA84652) and is assigned
the identifier CVE-2018-20030.