partition_snapshot_row_cursor: initialize _dummy and _continuous
partition_snapshot_row_cursor: initialize _dummy and _continuous

Debug mode view_schema_test sometimes complains that a bool member
doesn't contain in-range values, apparenty in the move constructor.

Initialize them for its benefit to avoid false-positive test
failures.
Message-Id: <0a1bf32715d41abba3600d2bac2243db2f76cfd7@scylladb.com>

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