When either HOME or XDG_CONFIG_HOME are unset getenv() can return NULL.
C++ strings don't like being set to NULL, so we need to check for this
case.
Bryan: Edited Sconstruct and commit message.
Signed-off-by: Josh Larson <theMutatedShrimp@gmail.com>
Signed-off-by: Bryan Schumaker <bjschuma@gmail.com>