Fix ReferenceError: localMediaStream

When 'use strict' is in effect, you cannot just assign to magic global
variables inside an anonymous function.

Define a local in the surrounding scope so that 'scan' and
'successCallback' can share it without relying on the global namespace.

Note: this bug affected only those using non-minified source, because
whatever minifier was used by the upstream to produce lib/html5-qrcode.min.js
stripped away the 'use strict' directive!