Fix GH-9248: Segmentation fault in mb_strimwidth()

We need to initialize the optional argument `trimmarker` with its
default value.

Closes GH-9273.

Author: cmb69

NEWS

@@ -6,6 +6,9 @@ PHP                                                                        NEWS
   . Fixed incorrect double to long casting in latest clang. (zeriyoshi)
   . Added support for defining constants in traits. (sj-i)
 
+- MBString:
+  . Fixed bug GH-9248 (Segmentation fault in mb_strimwidth()). (cmb)
+
 - Random:
   . Fixed bug GH-9235 (non-existant $sequence parameter in stub for
     PcgOneseq128XslRr64::__construct()). (timwolla)

ext/mbstring/mbstring.c

@@ -2502,7 +2502,7 @@ static zend_string* mb_trim_string(zend_string *input, zend_string *marker, cons
 /* Trim the string to terminal width; optional, add a 'trim marker' if it was truncated */
 PHP_FUNCTION(mb_strimwidth)
 {
-	zend_string *str, *trimmarker, *encoding = NULL;
+	zend_string *str, *trimmarker = zend_empty_string, *encoding = NULL;
 	zend_long from, width;
 
 	ZEND_PARSE_PARAMETERS_START(3, 5)

ext/mbstring/tests/gh9248.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Bug GH-9248 (Segmentation fault in mb_strimwidth())
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+var_dump(mb_strimwidth('The quick brown fox', 0, 8));
+?>
+--EXPECT--
+string(8) "The quic"