Fix AIX compiler warning

Fix AIX compiler warning indicating that variable may not be
initialized

	modified:   src/heap/heap.cc

R=danno@chromium.org, svenpanne@chromium.org, mbrandy@us.ibm.com

BUG=

Review URL: https://codereview.chromium.org/1112883002

Cr-Commit-Position: refs/heads/master@{#28145}