Fix another regression seeking PNG files¶

This fixes a regression introduced in 7.1.0 when adding support for APNG files.

When callingseek(n) on a regular PNG wheren>0, it failed to raise anEOFError as it should have done, resulting in:

AttributeError: 'NoneType' object has no attribute 'read'

Pillow 7.1.2 now raises the correct exception.