[BUG] [PLAYER] Non-NXP and LVL files can be loaded

[Wafflem]

Steps for the glitch to occur:
1. Open the NeoLemmix.exe
2. Type the name of any NXP you have played at least once, at the "File name" section.
3. Look at the list of suggestions. You will see files that are not of the LVL/NXP extension (see example picture below).
4. Choose a non LVL/NXP file. The player will then crash.

[namida]

I'm not really sure how far I want to go with preventing people loading invalid files, especially with the current situation being a temporary one. As long as it's not corrupting any data, but just crashing (at a point before anything even loads), I don't really think this matters.