AP_NavEKF3: fixed memory corruption on push before init

this fixes a bug that happens with VISION_SPEED_ESTIMATE from a
companion computer, which may come in before the EKF buffers are
allocated. That causes a push to an uninitialised ringbuffer which
triggers memory corruption

found using the new memory guard system

libraries/AP_NavEKF3/AP_NavEKF3_Buffer.h

@@ -85,6 +85,9 @@ public:
     */
     inline void push(element_type element)
     {
+        if (buffer == nullptr) {
+            return;
+        }
         // Advance head to next available index
         _head = (_head+1)%_size;
         // New data is written at the head