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
    
    (cherry picked from commit 85707cfdaddc179af8bd2623091eb1b8c58fed4a)