SecurityPkg: Fix assert when setting key from eMMC/SD/USB

SecurityPkg: Fix assert when setting key from eMMC/SD/USB

When secure boot is enabled, if one loads keys from a FAT formatted
eMMC/SD/USB when trying to provision PK/KEK/DB keys via the menu,
an assert in StrLen() occurs.
This is because the filename starts on odd address, which is not a uint16
aligned boundary: https://bugzilla.tianocore.org/show_bug.cgi?id=1003

There are further known issues with the OpenFileByDevicePath() function;
those are tracked by
<https://bugzilla.tianocore.org/show_bug.cgi?id=1008>.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Roman Bacik <roman.bacik@broadcom.com>
Reviewed-by: "Yao, Jiewen" <jiewen.yao@intel.com>
[lersek@redhat.com: whitespace fixes]
[lersek@redhat.com: reference TianoCore BZ#1008]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>