Add another test for GB18030 text conversion

Author: alexdowad

ext/mbstring/tests/gb18030_encoding.phpt

@@ -295,6 +295,8 @@ testInvalidString("\x90\x30\x81\xFF", "\x00\x00\x00%", "GB18030", "UTF-32BE");
 testInvalidString("\xE3\x32\x9A\x36", "\x00\x00\x00%", "GB18030", "UTF-32BE");
 testInvalidString("\xE4\x30\x81\x35", "\x00\x00\x00%\x00\x00\x00%", "GB18030", "UTF-32BE");
 
+testInvalidString("\x90\x30\x80\x30", "\x00\x00\x00%\x00\x00\x00\x30", "GB18030", "UTF-32BE");
+
 echo "Tested GB18030 4-byte characters <-> UTF-16BE\n";
 
 testAllValidChars($fromUnicode, 'UTF-16BE', 'GB18030', false);