MORTEVIELLE: Fix two GCC compiler warnings.

The first is due to a variable set but never used i.e. destSurface, but
I think this call is still needed to lock the area being copied, hence
have removed the variable, but not the call.

The second was a real bug in the type of the print format string used
for savegame naming. slot is a number, not a string and can vary from 0
to 999, hence the 3 digit fixed size with zero padded prefix.