Remove a double free(9) in xpt_bus_register

In xpt_bus_register(), remove superfluous call to free(). This was mostly
benign since free(9) checks for NULL before doing anything, and
xpt_create_path() is nice enough to NULL out the pointer on failure.
However, it could've segfaulted if malloc(9) failed during
xpt_create_path().

Submitted by: gibbs
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp