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

[quickfur]

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?

[yebblies]

A lot of dmd's driver code has zero test suite coverage.

[yebblies]

Auto-merge toggled on