Bug report

Bug summary

Reading a truncated png file can segfault the process.

Code for reproduction

from pylab import *
rcdefaults()
savefig("test.png")
buf = open("test.png", "rb").read()
open("test.png", "wb").write(buf[:20])  # truncate to first 20 bytes
imread("test.png")

Actual outcome

Fatal Python error: Segmentation fault

Current thread 0x00007f3638e8a540 (most recent call first):
  File "/usr/lib/python3.6/site-packages/matplotlib/image.py", line 1298 in imread
  File "/usr/lib/python3.6/site-packages/matplotlib/pyplot.py", line 2314 in imread
  File "test.py", line 6 in <module>
[1]    12789 segmentation fault (core dumped)  python test.py

Expected outcome

Some exception is raised.

(To be honest I'm not fully convinced there's a much of a benefit in shipping our own png wrappers rather than, say, relying on PIL throughout.)

Matplotlib version

• Operating System: Arch Linux
• Matplotlib Version: 2.0.2 and master
• Python Version: 3.6
• Jupyter Version (if applicable): NA
• Other Libraries: NA