ext/gettext: throw on libintl NULL return in textdomain/bindtextdomain (#21882)

Both functions can return NULL on libintl-internal allocation failure,
which RETURN_STRING then fed to strlen(NULL) and crashed. Throw an
Error in that case instead, leaving the existing string return type
intact (no static-analysis fallout from a widened signature).

The dir==NULL query path of bindtextdomain keeps its RETURN_FALSE
because musl returns NULL there to signal an unbound domain, which is
not an error condition.

Author: iliaal

ext/gettext/gettext.c

@@ -98,6 +98,11 @@ PHP_FUNCTION(textdomain)
 
 	retval = textdomain(domain_name);
 
+	if (UNEXPECTED(retval == NULL)) {
+		zend_throw_error(NULL, "Could not set text domain");
+		RETURN_THROWS();
+	}
+
 	RETURN_STRING(retval);
 }
 /* }}} */
@@ -211,6 +216,11 @@ PHP_FUNCTION(bindtextdomain)
 
 	retval = bindtextdomain(ZSTR_VAL(domain), dir_name);
 
+	if (UNEXPECTED(retval == NULL)) {
+		zend_throw_error(NULL, "Could not bind text domain");
+		RETURN_THROWS();
+	}
+
 	RETURN_STRING(retval);
 }
 /* }}} */