Display error if command array item (for proc_open) can't be converted to string

Author: alexdowad

ext/standard/proc_open.c

@@ -396,6 +396,7 @@ struct php_proc_open_descriptor_item {
 static zend_string *get_valid_arg_string(zval *zv, int elem_num) {
 	zend_string *str = zval_get_string(zv);
 	if (!str) {
+		zend_value_error("Command array element %d could not be converted to string", elem_num);
 		return NULL;
 	}