Fuzzing for the json decoder

Hey it seems like it's working!

fuzz/fuzz_targets/fuzz_target_1.rs

@@ -0,0 +1,11 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+extern crate fwd;
+use fwd::server::refresh::docker::JsonValue;
+
+fuzz_target!(|data: &[u8]| {
+    // fuzzed code goes here
+    let _ = JsonValue::parse(data);
+});