Fix another regression seeking PNG files#
This fixes a regression introduced in 7.1.0 when adding support for APNG files.
seek(n) on a regular PNG where
n > 0, it failed to raise an
EOFError as it should have done, resulting in:
AttributeError: 'NoneType' object has no attribute 'read'
Pillow 7.1.2 now raises the correct exception.