Fix sparc64 libc build after r320742.

p_branch_empty was declared but never used due to an oversight. Use it as
designed, further comment on its return value.

Reported by: Jenkins (head-sparc64)
Reviewed by: emaste
Approved by: emaste (mentor)
MFC with: r320742
Differential Revision: https://reviews.freebsd.org/D11506