VIM-855 Fix regexp character class bugs

Previously strchr and istrchr didn't consider NUL chars as end-of-string
terminators. This caused problems in regexps using character classes:
a regexp like "[^a]bc" would be effectively treated as "[^abc]bc" - i.e.
some literal characters from the rest of the pattern would accidentally be
included in the character class.
Merge PR-59, fixing VIM-855 Regexp character class bugs

* github.com:JetBrains/ideavim:
  VIM-855 Fix regexp character class bugs
Release notes and version files for build 0.39-jbyler-6