[asan] Avoid redundant poisoning checks in __sanitizer_contiguous_container_find_bad_address.

__sanitizer_contiguous_container_find_bad_address computes three regions of a
container to check for poisoning: begin, middle, end. The issue is that in current
design the first region can be significantly larger than kMaxRangeToCheck.

Proposed patch fixes a typo to calculate the first region properly.

Patch by Ivan Baravy.

Differential Revision: https://reviews.llvm.org/D27061

llvm-svn: 288234