bhyve: avoid updating fbaddr when vm_mmap_memseg fails

In the function pci_fbuf_baraddr the field sc->fbaddr was set with a
user-controlled value, even though the call to vm_mmap_memseg failed.

No security risk as currently sc->fbaddr is not really used in the
source code.

Reported by: Synacktiv
Reviewed by: emaste
Sponsored by: Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D46109