ext/gettext: return false from textdomain() and bindtextdomain() on NULL

Both functions can return NULL on libintl-internal allocation failure.
The PHP wrappers passed the result directly to RETURN_STRING, which
calls strlen(NULL) and segfaults. Mirrors 0221cee, which closed
the same gap for bindtextdomain($domain, NULL).

textdomain()'s return type widens from string to string|false.

Author: iliaal

ext/gettext/gettext.c

@@ -99,6 +99,9 @@ PHP_FUNCTION(textdomain)
 	}
 
 	retval = textdomain(domain_name);
+	if (retval == NULL) {
+		RETURN_FALSE;
+	}
 
 	RETURN_STRING(retval);
 }
@@ -212,6 +215,9 @@ PHP_FUNCTION(bindtextdomain)
 	}
 
 	retval = bindtextdomain(ZSTR_VAL(domain), dir_name);
+	if (retval == NULL) {
+		RETURN_FALSE;
+	}
 
 	RETURN_STRING(retval);
 }

ext/gettext/gettext.stub.php

@@ -3,7 +3,7 @@
 /** @generate-class-entries */
 
 /** @refcount 1 */
-function textdomain(?string $domain = null): string {}
+function textdomain(?string $domain = null): string|false {}
 
 /** @refcount 1 */
 function gettext(string $message): string {}