Fix: Stop processing if a file should be stored outside the dist-directory.

I discovered this case when building test-case basic/test_python_makefile
while in a virtual environment.

I decided to implement a hard stop, since PyInstaller must never ever
write outside the dist-directory. Otherwise an attacker could overwrite
arbitrary files. This is the classic file traversal vulnerability.

Still todo: fix the test-case basic/test_python_makefile to put the
file at the correct place.