Add test for iconv_strpos() internal encoding error

Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>

Closes GH-8816.

Author: jcm

ext/iconv/tests/iconv_strpos_error_001.phpt

@@ -0,0 +1,13 @@
+--TEST--
+iconv_strpos() - test against PHP's default internal encoding
+--EXTENSIONS--
+iconv
+--FILE--
+<?php
+$large_enconding = str_repeat("X", 256);
+ini_set("default_charset", $large_enconding);
+var_dump(iconv_strpos('Hello, world', 'world', -2));
+?>
+--EXPECTF--
+%AWarning: iconv_strpos(): Wrong encoding, conversion from %s
+bool(false)