Fix legacy conversion filter for GB18030

Author: alexdowad

ext/mbstring/libmbfl/filters/mbfilter_gb18030.c

@@ -364,6 +364,8 @@ int mbfl_filt_conv_wchar_gb18030(int c, mbfl_convert_filter *filter)
 
 	if (c == 0) {
 		s = 0;
+	} else if (s == 0) {
+		s = -1;
 	}
 
 	if (s >= 0) {

ext/mbstring/tests/gb18030_encoding.phpt

@@ -302,6 +302,10 @@ echo "Tested GB18030 4-byte characters <-> UTF-16BE\n";
 testAllValidChars($fromUnicode, 'UTF-16BE', 'GB18030', false);
 echo "Tested UTF-16BE -> GB18030 (1 and 2 byte characters)\n";
 
+// Regression test
+// This was found by a fuzzer (previously the invalid codepoint would be converted to \x00)
+convertInvalidString("\xAA\xB8\x2D\x38\x00\x00\x00#", "%#", "UTF-32BE", "GB18030");
+
 // Test "long" illegal character markers
 mb_substitute_character("long");
 convertInvalidString("\x81\x30\x81\xFF", "%", "GB18030", "UTF-8");