Fix potential PNG decompression DOS

@hugovk @aclark4life 

Two outstanding bits: 

1) interface for the developer -- It behaves a little differently than the decompression bomb warning in that this just tosses a value error. But it's less likely to have false positives.
2) It doesn't defend against images that have many just under the limit text segments. 

Opinions?

(note that I had to change the test to pass -- I believe that the new version is also a reasonable representation of an invalid iTXt segment)

Unused import sys