MFC r305365

The bug:
$ echo x | awk '/:cntrl:/'
x

The NUL character in cntrl class truncates the pattern, and an empty
pattern matches anything. The patch skips NUL as a quick fix.

PR: 195792
Submitted by: kdrakehp@zoho.com
Approved by: bwk@cs.princeton.edu (the author)