Fix GH-20833: mb_str_pad() divide by zero if padding string is invalid in the encoding

If the padding string is not valid in the given encoding,
mb_get_strlen() can return 0.

Author: ndossche

ext/mbstring/mbstring.c

@@ -5848,6 +5848,11 @@ PHP_FUNCTION(mb_str_pad)
 	}
 
 	size_t pad_length = mb_get_strlen(pad, encoding);
+	if (pad_length == 0) {
+		/* Possible with invalidly encoded padding string. */
+		zend_argument_must_not_be_empty_error(3);
+		RETURN_THROWS();
+	}
 
 	size_t num_mb_pad_chars = pad_to_length - input_length;
 

ext/mbstring/tests/gh20833.phpt

@@ -0,0 +1,14 @@
+--TEST--
+GH-20833 (mb_str_pad() divide by zero if padding string is invalid in the encoding)
+--FILE--
+<?php
+$utf8 = "test";
+$utf32 = mb_convert_encoding($utf8, 'UTF-32', 'UTF-8');
+try {
+    mb_str_pad($utf32, 5, "1" /* invalid for encoding */, STR_PAD_RIGHT, "UTF-32");
+} catch (ValueError $e) {
+    echo $e::class, ": ", $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+ValueError: mb_str_pad(): Argument #3 ($pad_string) must not be empty