[bug-gawk] Minor Gawk bug

[Chris Lauderdale]

I've tested on Linux vers. 3.0.0, 3.1.0, 4.0.0, 4.0.1, and 4.1.3, and only on 
the last one do I get an internal error for this, so it's at least recent.  
AFAICT it's engaged when one uses a regex range match that's ~incompatible with 
the current character set (per LC_CTYPE); in my case, LC_CTYPE=C.UTF-8 by 
default.  Goes away if LC_CTYPE=C, with option -b, or for just [\x80] so I'd 
guess it's trying to un-UTF8 the escapes in order to determine what the range 
"means" and failing. Minimal reproducer:

BEGIN { "" ~ /[\x80-\xBF]/ }

Might oughta be an error (though that would tie program validity to the current 
locale, the thought of which makes my skin crawl) but definitely shouldn't be 
an abort.