stack-uar.c is flaky (1 in 256 executions) because the random tag
may be zero (https://github.com/llvm/llvm-project/issues/69221).
This patch works around the issue in the same way as deep-recursion.c
(aa4dfd3736),
by falling back to a neighboring object, which must have a different
(non-zero) tag.
This patch also does a minor cleanup of the aforementioned
deep-recursion.c, for consistency with stack-uar.c.
Co-authored-by: Thurston Dang <thurston@google.com>
ddf1de20a3