getopt: fix fencepost error in ambiguous-W-option handling

getopt_long contains an undocumented (AFAICT) feature in which, if you
put "W;" in the short-options list, then '-W foo' and '-Wfoo' are
treated as equivalent to '--foo'.  This is implemented with a partial
second copy of the code for handling long options, and that code
increments optind one too many times when recovering from an ambiguous
abbreviated option, which can cause the main loop to walk past the end
of argv and crash.

I discovered this while writing a test case that tries to exercise all
of getopt's error reporting paths; I wouldn't be surprised to learn
that this feature is never used by real applications.

	* posix/getopt.c (_getopt_internal_r): Don't increment
	d->optind a second time when reporting ambiguous -W options.

ChangeLog

@@ -1,5 +1,8 @@
 2017-04-07  Zack Weinberg  <zackw@panix.com>
 
+	* posix/getopt.c (_getopt_internal_r): Don't increment
+	d->optind a second time when reporting ambiguous -W options.
+
 	* posix/getopt_int.h: Include getopt.h.
 	Use impl-namespace names for all arguments to _getopt_internal and
 	_getopt_internal_r.

posix/getopt.c

@@ -850,7 +850,6 @@ _getopt_internal_r (int argc, char *const *argv, const char *optstring,
 #endif
 	      }
 	    d->__nextchar += strlen (d->__nextchar);
-	    d->optind++;
 	    return '?';
 	  }
 	if (pfound != NULL)