Only close __fp if not fp

[radarhere]

See #3536

[hugovk]

Using the test files in #3536, this PR reverts the exception back to that in 5.3.0:

5.3.0

good.tiff <PIL.TiffImagePlugin.TiffImageFile image mode=RGB size=124x124 at 0x1085FFAC8>
Traceback (most recent call last):
  File "test.py", line 5, in <module>
    f3 = Image.open('irr_cubemap.tiff')
  File "/usr/local/lib/python3.7/site-packages/PIL/Image.py", line 2657, in open
    % (filename if filename else fp))
OSError: cannot identify image file 'irr_cubemap.tiff'

5.4.0

good.tiff <PIL.TiffImagePlugin.TiffImageFile image mode=RGB size=124x124 at 0x10A562AC8>
Traceback (most recent call last):
  File "test.py", line 5, in <module>
    f3 = Image.open('irr_cubemap.tiff')
  File "/usr/local/lib/python3.7/site-packages/PIL/Image.py", line 2676, in open
    im = _open_core(fp, filename, prefix)
  File "/usr/local/lib/python3.7/site-packages/PIL/Image.py", line 2657, in _open_core
    fp.seek(0)
ValueError: seek of closed file

This PR

good.tiff <PIL.TiffImagePlugin.TiffImageFile image mode=RGB size=124x124 at 0x1087B0C88>
Traceback (most recent call last):
  File "test.py", line 5, in <module>
    f3 = Image.open('irr_cubemap.tiff')
  File "/Users/hugo/github/Pillow/src/PIL/Image.py", line 2687, in open
    % (filename if filename else fp))
OSError: cannot identify image file 'irr_cubemap.tiff'