[ASan] make variable unsigned to silence the warning

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@175284 91177308-0d34-0410-b5e6-96231b3b80d8
[ASan] make variables unsigned to silence warnings - attempt 2

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@175285 91177308-0d34-0410-b5e6-96231b3b80d8