linux-user: don't crash with null name

From Thayne Harbaugh.

path() may be called with null string, don't bother trying to
remap in that case.

Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6480 c046a42c-6fe2-441c-8c8c-71466251a162
