[MMBotMDHacking] Fix a deadlock in Asm68kCommand

The ChangeDirectory() method uses a monitor to avoid multiple threads
from changing the current directory concurrently, and RestoreDirectory()
releases the monitor. Asm68kCommand did not use a try-finally block to
call RestoreDirectory(), so the lock could remain acquired if an
exception is thrown before RestoreDirectory() is called.
Merge pull request #3 from FraGag/fix-deadlock-asm68k

[MMBotMDHacking] Fix a deadlock in Asm68kCommand