MdeModulePkg/AtaPassThru: Add missing NULL ptr check in BindingStart

MdeModulePkg/AtaPassThru: Add missing NULL ptr check in BindingStart

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=916

Within function AtaAtapiPassThruStart():
Add missing NULL pointer check for variable 'Instance' under the
'ErrorExit' code logic.

Cc: Steven Shi <steven.shi@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>