nextFreeFast is currently not inlined by the compiler due
to its size and complexity. This CL simplifies
nextFreeFast by letting the slow path handle (nextFree)
handle a corner cases.

Change-Id: Ia9c5d1a7912bcb4bec072f5fd240f0e0bafb20e4
Reviewed-on: https://go-review.googlesource.com/22598Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>