Added fuzzer

Signed-off-by: AdamKorcz <adam@adalogics.com>

internal/db/fuzzer.go

@@ -0,0 +1,14 @@
+// Copyright 2021 The Gogs Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+// +build gofuzz
+
+package db
+
+func Fuzz(data []byte) int {
+	_, err := CheckPublicKeyString(string(data))
+	if err != nil {
+		return 0
+	}
+	return 1
+}