sim: fix minor --sysroot mem leak

sim: fix minor --sysroot mem leak

The current --sysroot parsing attempts to keep from leaking memory by
treating the empty string specially (sine this is the initial value),
but it ends up leaking memory when the arg is an empty string.  So if
someone uses --sysroot "", the old value is leaked, as is the new one.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>