AP_GPS: fixed constrained NaN in EKF3 caused by bad GPS blending

if the accuracies reported are very low then we can do a division by
zero and this results in a constraining NaN for GPS vertical velocity
filter in NavEKF3_core::calcGpsGoodToAlign

libraries/AP_GPS/AP_GPS.cpp

@@ -1541,6 +1541,10 @@ bool AP_GPS::calc_blend_weights(void)
         }
     }
 
+    if (!is_positive(sum_of_all_weights)) {
+        return false;
+    }
+
     // calculate an overall weight
     for (uint8_t i=0; i<GPS_MAX_RECEIVERS; i++) {
         _blend_weights[i] = (hpos_blend_weights[i] + vpos_blend_weights[i] + spd_blend_weights[i]) / sum_of_all_weights;