AC_WPNav: check validity of kP

libraries/AC_WPNav/AC_WPNav.cpp

@@ -386,7 +386,7 @@ bool AC_WPNav::advance_wp_target_along_track(float dt)
     // calculate point at which velocity switches from linear to sqrt
     float linear_velocity = _wp_speed_cms;
     float kP = _pos_control.get_pos_xy_p().kP();
-    if (kP >= 0.0f) {   // avoid divide by zero
+    if (is_positive(kP)) {   // avoid divide by zero
         linear_velocity = _track_accel/kP;
     }