AP_RSSI: handle init ordering with MSP

this fixes a nullptr dereference on startup when MSP asks for RSSI
data before the RSSI library is initialised

Fixes #15824

libraries/AP_RSSI/AP_RSSI.cpp

@@ -167,6 +167,9 @@ uint8_t AP_RSSI::read_receiver_rssi_uint8()
 // read the RSSI value from an analog pin - returns float in range 0.0 to 1.0
 float AP_RSSI::read_pin_rssi()
 {
+    if (!rssi_analog_source) {
+        return 0;
+    }
     rssi_analog_source->set_pin(rssi_analog_pin);
     float current_analog_voltage = rssi_analog_source->voltage_average();