don't let recently_chosen_ntors overflow

with commit c6f1668d we let it grow arbitrarily large.

it can still overflow, but the damage is very small now.

src/or/onion.c

@@ -212,7 +212,7 @@ static uint16_t
 decide_next_handshake_type(void)
 {
   /* The number of times we've chosen ntor lately when both were available. */
-  static int recently_chosen_ntors = 0;
+  static unsigned int recently_chosen_ntors = 0;
 
   if (!ol_entries[ONION_HANDSHAKE_TYPE_NTOR])
     return ONION_HANDSHAKE_TYPE_TAP; /* no ntors? try tap */