AP_TECS: ensure good TECS state before running update_pitch_throttle

update_pitch_throttle can be called when update_50hz hasn't run in a very long time, or ever.  This requires a main loop rate >50Hz, and for the mode change to occur in the same loop that update_50Hz doesn't run but update_pitch_throttle does.

libraries/AP_TECS/AP_TECS.cpp

@@ -1160,8 +1160,18 @@ void AP_TECS::update_pitch_throttle(int32_t hgt_dem_cm,
                                     float hgt_afe,
                                     float load_factor)
 {
-    // Calculate time in seconds since last update
     uint64_t now = AP_HAL::micros64();
+    // check how long since we last did the 50Hz update; do nothing in
+    // this loop if that hasn't run for some signficant period of
+    // time.  Notably, it may never have run, leaving _TAS_state as
+    // zero and subsequently division-by-zero errors.
+    const float _DT_for_update_50hz = (now - _update_50hz_last_usec) * 1.0e-6f;
+    if (_update_50hz_last_usec == 0 || _DT_for_update_50hz > 1.0) {
+        // more than 1 second since it was run, don't do anything yet:
+        return;
+    }
+
+    // Calculate time in seconds since last update
     _DT = (now - _update_pitch_throttle_last_usec) * 1.0e-6f;
     _DT = MAX(_DT, 0.001f);
     _update_pitch_throttle_last_usec = now;