Prevent searching from breaking when opening a new file in the web viewer

When testing #3607, I stumbled upon another search related bug. Steps to reproduce:
1. Open: http://mozilla.github.io/pdf.js/web/viewer.html.
2. Search for any word.
3. Click on the "Open File" button and load a new file.
4. Do a new search.

__Expected result:__ Searching should work in the new document.

__Actual result:__ Searching fails, with the following console message:
`[12:38:33.353] TypeError: this.extractTextPromises[i] is undefined @ http://mozilla.github.io/pdf.js/web/viewer.js:794`

This PR fixes the above by resetting a couple of parameters in `PDFFindController` whenever a file is loaded, thus preventing find from using data from a previously loaded file when the user searches.