The simulation platform radio driver doesn't set the 'mInfo.mTxInfo.mIeInfo'
field of the ACK frame. And the function 'otMacFrameUpdateTimeIe()' directly
use the 'mInfo.mTxInfo.mIeInfo' field, this may cause the program crash.
This commit checks whether 'mInfo.mTxInfo.mIeInfo' is null before using it
and sets the 'mInfo.mTxInfo.mIeInfo' field of the ACK frame to null in
simulation platform.