standard: Fix error check for proc_open() command

zval_get_string() can never return NULL, you need to use the try version
to get NULL. This is observable because the process will still spawn
even if an exception had occurred. To fix this, use the try variant.

Closes GH-20650.

NEWS

@@ -16,6 +16,9 @@ PHP                                                                        NEWS
 - LDAP:
   . Fix memory leak in ldap_set_options(). (ndossche)
 
+- Standard:
+  . Fix error check for proc_open() command. (ndossche)
+
 18 Dec 2025, PHP 8.3.29
 
 - Core:

ext/standard/proc_open.c

@@ -510,7 +510,7 @@ typedef struct _descriptorspec_item {
 } descriptorspec_item;
 
 static zend_string *get_valid_arg_string(zval *zv, int elem_num) {
-	zend_string *str = zval_get_string(zv);
+	zend_string *str = zval_try_get_string(zv);
 	if (!str) {
 		return NULL;
 	}