Catch struct.error from truncated EXIF when reading JPEG DPI

[radarhere]

Resolves #7456

The image in the issue raises a UnidentifiedImageError. Hiding behind that is the following traceback when attempting to read DPI from JPEG EXIF.

Traceback (most recent call last):
  File "PIL/ImageFile.py", line 117, in __init__
  File "PIL/JpegImagePlugin.py", line 382, in _open
  File "PIL/JpegImagePlugin.py", line 160, in APP
  File "PIL/Image.py", line 1441, in getexif
  File "PIL/Image.py", line 3730, in load
  File "PIL/TiffImagePlugin.py", line 526, in __init__
  File "PIL/TiffImagePlugin.py", line 670, in _unpack
struct.error: unpack requires a buffer of 4 bytes

This PR adds struct.error to the list of exceptions caught, to handle this truncated data. The test image comes from the issue.