win/share/tileset.c seems only to be used by the MSDOS port, but it
compiles cleanly on OSX after these changes.
A file pointer was passed to fclose() twice, second time potentially
causing problems. There were cases of potentially null pointers
being passed to free() too. That should be safe these days, but it's
something we've tried to hard to avoid and would probably trigger
complaints from our own MONITOR_HEAP code if that ever got applied
here.
