png: initialize _n_frames

[pmrowla]

Fixes #4518

Changes proposed in this pull request:

• after the original APNG changes, ImageFile._seek_check() won't raise EOF for out of bounds seek unless _n_frames is properly set for standard (non-animated) PNG images

[pmrowla]

scikit-image with Pillow from PR branch:

scikit-image git:master  py:scikit-image ❯ pip freeze|egrep "Pillow|scikit-image"
Pillow==7.2.0.dev0
scikit-image==0.16.2

scikit-image git:master  py:scikit-image ❯ python
Python 3.7.7 (default, Mar 10 2020, 15:43:33)
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from skimage import data
>>> data.camera().astype('float')
array([[156., 157., 160., ..., 152., 152., 152.],
       [156., 157., 159., ..., 152., 152., 152.],
       [158., 157., 156., ..., 152., 152., 152.],
       ...,
       [121., 123., 126., ..., 121., 113., 111.],
       [121., 123., 126., ..., 121., 113., 111.],
       [121., 123., 126., ..., 121., 113., 111.]])
>>>

(using Pillow 7.1.1 or master reproduces #4518 )

[codecov]

Codecov Report

Merging #4519 into master will decrease coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #4519     +/-   ##
=========================================
- Coverage   89.81%   89.62%   -0.2%     
=========================================
  Files         329      327      -2     
  Lines       40898    40891      -7     
=========================================
- Hits        36731    36647     -84     
- Misses       4167     4244     +77

Flag	Coverage Δ
#AppVeyor	?
#GHA_Docker	85.88% <100%> (-0.04%)	⬇️
#GHA_Ubuntu	87.99% <100%> (-0.02%)	⬇️
#GHA_Windows	89.92% <100%> (-1.07%)	⬇️
#GHA_macOS	86.99% <100%> (-0.03%)	⬇️
#TravisCI	92.49% <100%> (-0.18%)	⬇️

Impacted Files	Coverage Δ
Tests/test_file_png.py	100% <100%> (ø)	⬆️
src/PIL/PngImagePlugin.py	91.88% <100%> (+0.01%)	⬆️
Tests/test_imagefont_bitmap.py	91.3% <0%> (-8.7%)	⬇️
src/PIL/PyAccess.py	82.48% <0%> (-7.35%)	⬇️
Tests/test_lib_pack.py	94.3% <0%> (-5.7%)	⬇️
Tests/test_image_quantize.py	92.45% <0%> (-5.67%)	⬇️
Tests/test_features.py	96.92% <0%> (-3.08%)	⬇️
src/PIL/ImageTk.py	73.59% <0%> (-2.41%)	⬇️
src/PIL/features.py	95.78% <0%> (-2.11%)	⬇️
Tests/test_file_webp.py	98% <0%> (-2%)	⬇️
... and 19 more

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[pmrowla]

CI error is some issue w/building qt on travis for the tox/lint step and is unrelated to these changes

[pmrowla]

closing in favor of #4528