[XLA:FFI] Fix annotating buffer `dimensions` memory
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[XLA:FFI] Fix annotating buffer `dimensions` memory

The last dimension of FFI buffers `dimensions` member variable was not annotated as initialized memory, because of an off-by-one code error.
It went unnoticed because of usage absl::c_accumulate algorithm in tests. Modified one test to directly access dimensions in a for loop, cause that triggers sanitizer check.
