don't use unitialized palette memory

I ran openxcom through valgrind and was confronted with a multitude of "Conditional jump or move depends on uninitialised value(s)" warnings.  I tracked a good number of them down to uninitialized palette memory.  I will do a more thorough examination of the remaining valgrind warnings, but the solutions to these are simple.

I looked into the SDL documentation and struct definitions to see exactly what wasn't getting initialized, and for some of the issues, it was just the Uint8 unused field of the SDL_Color struct.  Despite it's name, though, something in sdl is reading and making decisions based on the value of this field.