getopt_long(3): fix case of malformed long opt

When presented with an arg string like '-l-', getopt_long will successfully
parse out the 'l' short option, then proceed to match '--' against the first
longopts entry as it later does a strncmp with len=0. This latter bit is
arguably another bug in itself, but presumably not a practical issue as all
callers of parse_long_options are already doing the right thing (except this
one pointed out).

An opt string like '-l-' should be considered malformed and throw a bad
argument rather than behaving as if '--' were passed. It cannot possibly do
what the invoker expects, and it's probably the result of a typo (ls -l- a)
rather than any intent.

Reported by:	Tony Overfield <toverfield@yahoo.com>
Reviewed by:	imp
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D18616

lib/libc/stdlib/getopt_long.c

@@ -481,6 +481,8 @@ getopt_internal(int nargc, char * const *nargv, const char *options,
 #endif
 		if (*place == '-') {
 			place++;		/* --foo long option */
+			if (*place == '\0')
+				return (BADARG);	/* malformed option */
 #ifdef GNU_COMPATIBLE
 			dash_prefix = DD_PREFIX;
 #endif