Revision 305450

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)