Fix warnings in current debian-hardened CI.

We're getting "fallback annotation annotation in unreachable code"
warnings when we build with ALL_BUGS_ARE_FATAL. This patch fixes
that.

Fixes bug 40241.  Bugfix on 0.3.5.4-alpha.

changes/40241

+  o Minor bugfixes (compilation):
+    - Fix a compilation warning about unreachable fallthrough annotations
+      when building with "--enable-all-bugs-are-fatal" on some compilers.
+      Fixes bug 40241; bugfix on 0.3.5.4-alpha.

src/core/or/channeltls.c

@@ -1225,8 +1225,9 @@ channel_tls_handle_var_cell(var_cell_t *var_cell, or_connection_t *conn)
        * the v2 and v3 handshakes. */
       /* But that should be happening any longer've disabled bufferevents. */
       tor_assert_nonfatal_unreached_once();
-
+#ifndef ALL_BUGS_ARE_FATAL
       FALLTHROUGH;
+#endif
     case OR_CONN_STATE_TLS_SERVER_RENEGOTIATING:
       if (!(command_allowed_before_handshake(var_cell->command))) {
         log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,

src/core/or/circuitlist.c

@@ -787,7 +787,9 @@ circuit_purpose_to_controller_hs_state_string(uint8_t purpose)
              "Unrecognized circuit purpose: %d",
              (int)purpose);
       tor_fragile_assert();
+#ifndef ALL_BUGS_ARE_FATAL
       FALLTHROUGH;
+#endif
 
     case CIRCUIT_PURPOSE_OR:
     case CIRCUIT_PURPOSE_C_GENERAL:

src/feature/client/entrynodes.c

@@ -2263,7 +2263,9 @@ entry_guards_note_guard_success(guard_selection_t *gs,
       break;
     default:
       tor_assert_nonfatal_unreached();
+#ifndef ALL_BUGS_ARE_FATAL
       FALLTHROUGH;
+#endif
     case GUARD_CIRC_STATE_USABLE_IF_NO_BETTER_GUARD:
       if (guard->is_primary) {
         /* XXXX #20832 -- I don't actually like this logic. It seems to make

src/feature/rend/rendclient.c

@@ -819,7 +819,9 @@ rend_client_report_intro_point_failure(extend_info_t *failed_intro,
         log_warn(LD_BUG, "Unknown failure type %u. Removing intro point.",
                  failure_type);
         tor_fragile_assert();
+#ifndef ALL_BUGS_ARE_FATAL
         FALLTHROUGH;
+#endif
       case INTRO_POINT_FAILURE_GENERIC:
         rend_cache_intro_failure_note(failure_type,
                                       (uint8_t *)failed_intro->identity_digest,