Set is_padding_timer_scheduled to 0

When calling `circpad_send_padding_cell_for_callback()`, the flag
`is_padding_timer_scheduled` was not resetted to 0 which caused an issue
in the circpad simulator.
This commit fixes this problem.

Fixes #32671.

changes/bug32671

+  o Minor bugfixes (circuit padding):
+    - When circpad_send_padding_cell_for_callback is called,
+      `is_padding_timer_scheduled` flag was not reset. Now it is set to 0 at
+      the top of that function. Fix bug #32671.

src/core/or/circuitpadding.c

@@ -1226,6 +1226,7 @@ circpad_send_padding_cell_for_callback(circpad_machine_runtime_t *mi)
   circuit_t *circ = mi->on_circ;
   int machine_idx = mi->machine_index;
   mi->padding_scheduled_at_usec = 0;
+  mi->is_padding_timer_scheduled = 0;
   circpad_statenum_t state = mi->current_state;
 
   /* Make sure circuit didn't close on us */