[Trunk] Failure to locate/open MP3 library causes crash

1. Open score.
2. Move lame_enc.dll somewhere else (if you've previously set the location in MuseScore, with success).
3. Export as MP3.
4. When you are prompted about locating the LAME library, select Yes.
5. Select a lame_enc.dll.

Result: Dialogue appears with 'Could not open MP3 encoding library!' and crashes.

Discussion: I may have had one lame_enc.dll it didn't happen to (I have software that uses different ones in their folders - judging by the size), but I don't remember the result.

Using MuseScore 2.0 nightly build (4242) - Windows XP SP3.