When a new tiny block is allocated because we're allocating an object
that won't fit into the current block, mallocgc saves the new block if
it has more space leftover than the old block.  However, the logic for
this was subtly broken in golang.org/cl/2814, resulting in never
saving (or consequently reusing) a tiny block.

Change-Id: Ib5f6769451fb82877ddeefe75dfe79ed4a04fd40
Reviewed-on: https://go-review.googlesource.com/16330
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>