vm_phys_avail_count(): Fix out-of-bounds accesses

On improper termination of phys_avail[] (two consecutive 0 starting at
an even index), this function would (unnecessarily) continue searching
for the termination markers even if the index was out of bounds.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48629