[Asan] Return nullptr for invalid chunks

CHUNK_ALLOCATED. CHUNK_QUARANTINE are only states
which make AsanChunk useful for GetAsanChunk callers.
In either case member of AsanChunk are not useful.

Fix few cases which didn't expect nullptr. Most of the callers are already
expects nullptr.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D87135