parse_assert() accepts an optional `message` parameter that defaults
to nullptr. When the assertion fails and message is nullptr, it is
implicitly converted to sstring via the sstring(const char*) constructor,
which calls strlen(nullptr) -- undefined behavior that manifests as a
segfault in __strlen_evex.
This turns what should be a graceful malformed_sstable_exception into a
fatal crash. In the case of CUSTOMER-279, a corrupt SSTable triggered
parse_assert() during streaming (in continuous_data_consumer::
fast_forward_to()), causing a crash loop on the affected node.
Fix by guarding the nullptr case with a ternary, passing an empty
sstring() when message is null. on_parse_error() already handles
the empty-message case by substituting "parse_assert() failed".
Fixes: SCYLLADB-1329
Closesscylladb/scylladb#29285
cfebe17592