AP_BattMonitor: Limit the scope of possible corruption if the NeoDesigns gets a bad cell count

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_NeoDesign.cpp

@@ -24,7 +24,7 @@ void AP_BattMonitor_SMBus_NeoDesign::timer()
     // Get the cell count once, it's not likely to change in flight
     if (_cell_count == 0) {
         if (read_word(BATTMONITOR_ND_CELL_COUNT, data)) {
-            _cell_count = data;
+            _cell_count = MIN(data, max_cell_count); // never read in more cells then we can store
         } else {
             return; // something wrong, don't try anything else
         }

libraries/AP_BattMonitor/AP_BattMonitor_SMBus_NeoDesign.h

@@ -15,4 +15,6 @@ private:
     void timer(void) override;
 
     uint8_t _cell_count;
+
+    static const constexpr uint8_t max_cell_count = 10;
 };