Avoid conflict with non-json storage content in cmd/app/main.go

When running wasm-server on this project, if storage already has some items which content isn't JSON, it will conflict.

Web Console Log:

running wasm_exec.js:47:6
panic: invalid character 'v' looking for beginning of value wasm_exec.js:47:6
wasm_exec.js:47:6
goroutine 1 [running]: wasm_exec.js:47:6
main.main() wasm_exec.js:47:6
/tmp/go-wasm/dom/cmd/app/main.go:97 +0x6e

dennwc / dom

Avoid conflict with non-json storage content in cmd/app/main.go #36