Analyze minimum needed number of null separators in PngChunk::parseTXTChunk

This commit fixes the heap-buffer-overflow in PngChunk::parseTXTChunk.

According to the specification:
http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html

There must be 2 null separators when we start to analyze the language tag.

tests/bugfixes/github/test_CVE_2018_10999.py

@@ -13,5 +13,5 @@ class TestCvePoC(metaclass=system_tests.CaseMeta):
     stdout = [""]
     stderr = [
         """$exception_in_extract """ + filename + """:
-$kerFailedToReadImageData
+$kerCorruptedMetadata
 """]