Fix value of {kAvailableSpace} to prevent buffer growing.

Bug: v8:6600
Change-Id: Ifc1c3e191929db9e20aa302b2f52d450fba54909
Reviewed-on: https://chromium-review.googlesource.com/930881
Commit-Queue: Sreten Kovacevic <sreten.kovacevic@mips.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51460}