should use mkstemp for temporary files

Please fix flasm to use mkstemp for temporary files:

Forwarding http://bugs.debian.org/530383

It appears that flasm uses a temporary file "flasm.tmp"
in the current working directory while decompressing
a .swf, without checking to see if it already exists.
If a user already has a file with that name in the
current directory, it is overwritten without notice.

I guess having an important file called "flasm.tmp" isn't
particularly likely to happen, but I think this should
be fixed nevertheless, perhaps by using mkstemp?