Check for NULL when looking up getenv("HOME").

Fixes: https://issues.dlang.org/show_bug.cgi?id=12130

This bug is caused by dmd attempting to look up a literal '~' in `$PATH`, but when `HOME` is not set, `getenv("HOME")` returns NULL, which causes `strlen` to segfault. Note that this only happens if the user's `$PATH` environment variable contains a literal `~`; usually, in bash, attempting to set `~` in HOME will cause the `~` to be autoexpanded to the value of HOME, so to reproduce this bug, you have to escape the `~`, like so:
````
export PATH=\~:$PATH
unset HOME
dmd  # <--- segfaults
````

Question: This bug requires a specific runtime environment for dmd; does the testsuite provide facilities for testing this sort of thing?