MdeModulePkg/UefiBootManagerLib: BmGetActiveConsoleIn code cleanup

MdeModulePkg/UefiBootManagerLib: BmGetActiveConsoleIn code cleanup

Check for NULL from AllocateCopyPool before setting Count to 1. Also
change sizeof (EFI_HANDLE*) to sizeof (EFI_HANDLE).  Handles is a
EFI_HANDLE pointer, so the allocated memory must be the size of
EFI_HANDLE.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Thomas Palmer <thomas.palmer@hpe.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>