Malformed URLs no longer cause a crash

We're running `st` in production and had a bot hit our server with a bunch of bad URLs that caused st to crash. Here's the stack trace:

```json
"stack": [
    "URIError: URI malformed",
    "    at decodeURIComponent (native)",
    "    at Mount.getPath (/opt/app/node_modules/st/st.js:162:7)",
    "    at Mount.serve (/opt/app/node_modules/st/st.js:190:16)",
    "    at dispatch (/opt/app/node_modules/union/lib/routing-stream.js:110:21)",
    "    at RoutingStream.route (/opt/app/node_modules/union/lib/routing-stream.js:121:5)",
    "    at g (events.js:180:16)",
    "    at EventEmitter.emit (events.js:117:20)",
    "    at IncomingMessage.Readable.pipe (_stream_readable.js:548:8)",
    "    at Server.requestHandler (/opt/app/node_modules/union/lib/core.js:50:9)",
    "    at Server.EventEmitter.emit (events.js:98:17)"
  ]
```

The problem appears to be that a malformed URL causes an uncaught exception when `decodeURIComponent` fails. This solves the problem simply with a try/catch.