Switch an int32_t bin to a circpad_hist_index_t.

src/core/or/circuitpadding.c

@@ -293,7 +293,7 @@ circpad_histogram_usec_to_bin(const circpad_machine_state_t *mi,
 {
   const circpad_state_t *state = circpad_machine_current_state(mi);
   circpad_delay_t rtt_add_usec = 0;
-  int32_t bin;
+  circpad_hist_index_t bin;
 
   /* Our state should have been checked to be non-null by the caller
    * (circpad_machine_remove_token()) */

src/core/or/circuitpadding.h

@@ -235,6 +235,9 @@ typedef uint16_t circpad_statenum_t;
  *
  * Memory concerns are not so great here since the corresponding histogram and
  * histogram_edges arrays are global and not per-circuit.
+ *
+ * If we ever upgrade this to a value that can't be represented by 8-bits we
+ * also need to upgrade circpad_hist_index_t.
  */
 #define CIRCPAD_MAX_HISTOGRAM_LEN (100)