Bugfix: memory leak in f_execute_*_environment() functions, handle no-slash case...

Bugfix: memory leak in f_execute_*_environment() functions, handle no-slash case, fix memcpy()

I failed to clear memory on some return on error cases.

Using "sizeof(f_string_length) *" in memcpy is nonsense here, its a char, which is size 1!

When there is no '/' in the path, strrchr() return 0.
In this situation, just consider the entire path the file name.