streamed_mutation: make emit_range_tombstone() exception safe
streamed_mutation: make emit_range_tombstone() exception safe

For a time range tombstone that was already removed from a tree
is owned by a raw pointer. This doesn't end well if creation of
a mutation fragment or a call to push_mutation_fragment() throw.
Message-Id: <54c1727178de81eb1c3b5a4bd3de05765f19c80b@scylladb.com>

scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla