MdeModulePkg: TerminalDxe: avoid checking uninitialized variable

MdeModulePkg: TerminalDxe: avoid checking uninitialized variable

The SerialIo->GetControl() function is not required to set the
Control output parameter on error. Make sure we apply the
EFI_SERIAL_INPUT_BUFFER_EMPTY optimization in
TerminalConInTimerHandler() only if the SerialIo->GetControl()
function call set that bit in the Control variable.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18962 6f19259b-4bc3-4df7-8a09-765794883524