untyped_result_set_row's cell data type is bytes_opt, and the
get_block() accessor accesses the value assuming it's engaged
(relying on the caller to call has()).
has_unsalted_hash() calls get_blob() without calling has() beforehand,
potentially triggering undefined behavior.
Fix by using get_or() instead, which also simplifies the caller.
I observed failures in Jenkins in this area. It's hard to be sure
this is the root cause, since the failures triggered an internal
consistency assertion in asan rather than an asan report. However,
the error is hard to reproduce and the fix makes sense even if it
doesn't prevent the error.
See #3480 for the asan error.
Fixes#3480 (hopefully).
Message-Id: <20180602181919.29204-1-avi@scylladb.com>
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