AP_BattMonitor: Fix nullptr deref when resetting a battery

libraries/AP_BattMonitor/AP_BattMonitor.cpp

@@ -77,7 +77,6 @@ const AP_Param::GroupInfo AP_BattMonitor::var_info[] = {
 //
 AP_BattMonitor::AP_BattMonitor(uint32_t log_battery_bit, battery_failsafe_handler_fn_t battery_failsafe_handler_fn, const int8_t *failsafe_priorities) :
     _log_battery_bit(log_battery_bit),
-    _num_instances(0),
     _battery_failsafe_handler_fn(battery_failsafe_handler_fn),
     _failsafe_priorities(failsafe_priorities)
 {
@@ -557,11 +556,16 @@ void AP_BattMonitor::checkPoweringOff(void)
 */
 bool AP_BattMonitor::reset_remaining(uint16_t battery_mask, float percentage)
 {
+    static_assert(AP_BATT_MONITOR_MAX_INSTANCES <= 16, "More batteries are enabled then can be reset");
     bool ret = true;
     Failsafe highest_failsafe = Failsafe::None;
     for (uint8_t i = 0; i < _num_instances; i++) {
         if ((1U<<i) & battery_mask) {
+            if (drivers[i] != nullptr) {
                 ret &= drivers[i]->reset_remaining(percentage);
+            } else {
+                ret = false;
+            }
         }
         if (state[i].failsafe > highest_failsafe) {
             highest_failsafe = state[i].failsafe;