AP_Baro: optimize DroneCAN subscription process

* remove unnecessary nullptr check, these are always called from an
  initialized AP_DroneCAN so if it's nullptr something has gone
  horrifically wrong

* pass in driver index instead of repeatedly calling function to get it

* simplify error handling; knowing exactly which allocation failed is not
  super helpful and one failing likely means subsequent ones will too,
  as it can only fail due to being out of memory

libraries/AP_Baro/AP_Baro_DroneCAN.cpp

@@ -21,18 +21,13 @@ AP_Baro_DroneCAN::AP_Baro_DroneCAN(AP_Baro &baro) :
     AP_Baro_Backend(baro)
 {}
 
-void AP_Baro_DroneCAN::subscribe_msgs(AP_DroneCAN* ap_dronecan)
+bool AP_Baro_DroneCAN::subscribe_msgs(AP_DroneCAN* ap_dronecan)
 {
-    if (ap_dronecan == nullptr) {
-        return;
-    }
-    if (Canard::allocate_sub_arg_callback(ap_dronecan, &handle_pressure, ap_dronecan->get_driver_index()) == nullptr) {
-        AP_BoardConfig::allocation_error("pressure_sub");
-    }
+    const auto driver_index = ap_dronecan->get_driver_index();
 
-    if (Canard::allocate_sub_arg_callback(ap_dronecan, &handle_temperature, ap_dronecan->get_driver_index()) == nullptr) {
-        AP_BoardConfig::allocation_error("temperature_sub");
-    }
+    return (Canard::allocate_sub_arg_callback(ap_dronecan, &handle_pressure, driver_index) != nullptr)
+        && (Canard::allocate_sub_arg_callback(ap_dronecan, &handle_temperature, driver_index) != nullptr)
+    ;
 }
 
 AP_Baro_Backend* AP_Baro_DroneCAN::probe(AP_Baro &baro)

libraries/AP_Baro/AP_Baro_DroneCAN.h

@@ -15,7 +15,7 @@ class AP_Baro_DroneCAN : public AP_Baro_Backend {
 
     void update() override;
 
-    static void subscribe_msgs(AP_DroneCAN* ap_dronecan);
+    static bool subscribe_msgs(AP_DroneCAN* ap_dronecan);
     static AP_Baro_DroneCAN* get_dronecan_backend(AP_DroneCAN* ap_dronecan, uint8_t node_id, bool create_new);
     static AP_Baro_Backend* probe(AP_Baro &baro);