AP_GPS: fixed handling of driver destruction

this allows for more complex destructors, and ensures we don't have
multiple backends allocated at once per instance

libraries/AP_GPS/AP_GPS.cpp

@@ -182,9 +182,6 @@ AP_GPS::detect_instance(uint8_t instance)
 
 	if (new_gps != NULL) {
         state[instance].status = NO_FIX;
-        if (drivers[instance] != NULL) {
-            delete drivers[instance];
-        }
         drivers[instance] = new_gps;
         timing[instance].last_message_time_ms = now;
 	}
@@ -222,6 +219,10 @@ AP_GPS::update_instance(uint8_t instance)
         if (tnow - timing[instance].last_message_time_ms > 1200) {
             state[instance].status = NO_GPS;
             timing[instance].last_message_time_ms = tnow;
+            // free the driver before we run the next detection, so we
+            // don't end up with two allocated at any time
+            delete drivers[instance];
+            drivers[instance] = NULL;
         }
     } else {
         timing[instance].last_message_time_ms = tnow;

libraries/AP_GPS/GPS_Backend.h

@@ -25,6 +25,10 @@ class AP_GPS_Backend
 public:
 	AP_GPS_Backend(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDriver *_port);
 
+    // we declare a virtual destructor so that GPS drivers can
+    // override with a custom destructor if need be.
+    virtual ~AP_GPS_Backend(void) {}
+
     // The read() method is the only one needed in each driver. It
     // should return true when the backend has successfully received a
     // valid packet from the GPS.