Fix crash in active defrag (#883)

This crash would happen if we disable active defrag while it is running,
then re-enable active defrag. In this case the `expires_cursor` variable
in `activeDefragCycle()` would not be reset to 0 when disabling active
defrag, so when re-enabling it, this variable can still be non-zero,
which causes the `db` and other variables to not be initialized before
starting the defrag process.

Signed-off-by: Pierre Turin <pieturin@amazon.com>

src/defrag.c

@@ -941,6 +941,7 @@ void activeDefragCycle(void) {
             defrag_later_cursor = 0;
             current_db = -1;
             cursor = 0;
+            expires_cursor = 0;
             db = NULL;
             goto update_metrics;
         }