Don't use asprintf() in exists() as there may not be enough space
(fullname can become longer in find_file as names are unmangled).
This caused a very nasty SIGSEGV when trying to delete a mangled name.
Change to cheaper allocation on the stack with NAME_MAX+1 bytes for
the filename component.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@759 7758b857-130e-0410-ab9c-d8735a528d18