Description:
------------
Tiff Tag SubIFD (330) ONLY works is there is a single SubIFD, but the Tiff 
specifications state that SubIFDs is an array of IFDs of size tag_count.

If num_sub_ifd > 1, the tag long value is an offset to a sequence of longs, 
themselves representing the offsets of each SubIFD.

In the source (ext/exif/exif.c) method exif_process_IFD_in_TIFF assumes that the 
tag long value always points to ONE SubIFD (lines 3693 and 3698).


Effects:
1) If there is more than 1 SubIFD, the tiff parser will incorrectly be jumping 
around in the file, potentially printing TONS of warnings
2) exif_thumbnail() does NOT work for Nikon nef files


Test script:
---------------
Download the 2 nef files (nikon nefs are tiff files) below:
http://nefarious.timotheegroleau.com/tests/dsc_2317.nef
http://nefarious.timotheegroleau.com/tests/out.nef

Run the following scripts from a php 5.3.6:

php -r 'var_dump(exif_read_data("dsc_2317.nef"));' 2>&1 |less
php -r 'var_dump(exif_read_data("out.nef"));' 2>&1 |less

Notice the single warning for dsc_2317.nef, as the parser is quickly encountering an invalid entry, but notice also the ~39k warnings spat out for out.nef.


Note: I recommend enabling EXIF_DEBUG for this, to see the offsets printed (note btw that there's a compilation error with EXIF_DEBUG on, which I've commented out for my tests)


Expected result:
----------------
exif data returned, with no warnings when the input tiff file has multiple 
SubIFDs

Actual result:
--------------
exif data is returned, but based on the the input tiff, and where the SubIFD 
offset sends the parser, there can be *A LOT* of warnings being thrown.