QmakeParser: Do not leak temporaries into cache of files

This prevents an access to free'd memory when opening qtcreator.pro.

Looks like qml2puppet.pro gets added to that cache in QMakeVfs::writeFile
with part of the including pro-file's filename in it. That part gets
cleaned when that containing ProFile goes out of scope, leaving a key in
QMakeVfs::m_files free'd but accessible.

Change-Id: I80b43d2fbb66c214647497ea97e6e3a587e274d6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>