accept null pointer as message argument to gettext functions

the change to support passing null was rejected in the past on the
grounds that GNU gettext documented it as undefined, on an assumption
that only glibc accepted it and that the standalone GNU gettext did
not. but it turned out that both explicitly accept it.

in light of this, since some software assumes null can be passed
safely, allow it.

src/locale/dcngettext.c

@@ -132,6 +132,9 @@ char *dcngettext(const char *domainname, const char *msgid1, const char *msgid2,
 	struct binding *q;
 	int old_errno = errno;
 
+	/* match gnu gettext behaviour */
+	if (!msgid1) goto notrans;
+
 	if ((unsigned)category >= LC_ALL) goto notrans;
 
 	if (!domainname) domainname = __gettextdomain();