[ASan] Clear allocation magic value before recycling the chunk. This led to spurious crashes in LSan when it walked through reused chunks. Don't know how to create not-brittle test case for this.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@184404 91177308-0d34-0410-b5e6-96231b3b80d8