Merge branch 'trove_2020_004_041_v2' into maint-0.4.1

changes/ticket33619

+  o Major bugfixes (circuit padding, memory leaks):
+    - Avoid a remotely triggered memory leak in the case that a circuit
+      padding machine is somehow negotiated twice on the same circuit. Fixes
+      bug 33619; bugfix on 0.4.0.1-alpha.  Found by Tobias Pulls.  This is
+      also tracked as TROVE-2020-004.

src/core/or/circuitpadding.c

@@ -2381,9 +2381,12 @@ circpad_setup_machine_on_circ(circuit_t *on_circ,
     return;
   }
 
-  tor_assert_nonfatal(on_circ->padding_machine[machine->machine_index]
-                      == NULL);
-  tor_assert_nonfatal(on_circ->padding_info[machine->machine_index] == NULL);
+  IF_BUG_ONCE(on_circ->padding_machine[machine->machine_index] != NULL) {
+    return;
+  }
+  IF_BUG_ONCE(on_circ->padding_info[machine->machine_index] != NULL) {
+    return;
+  }
 
   /* Log message */
   if (CIRCUIT_IS_ORIGIN(on_circ)) {