AP_NavEKF: correct initialisation of ekf_imu_buffer

this init() call can be called on an existing buffer, in which case we clear the object.

Presumably since we've just zeroed all the elements its safe to say that we should mark the object as having never-been-filled

libraries/AP_NavEKF/EKF_Buffer.cpp

@@ -137,6 +137,7 @@ bool ekf_imu_buffer::init(uint32_t size)
     _size = size;
     _youngest = 0;
     _oldest = 0;
+    _filled = 0;
     return true;
 }