MdeModulePkg/UsbKbDxe: don't assert when the key read is invalid

MdeModulePkg/UsbKbDxe: don't assert when the key read is invalid

The GetKeyDescriptor() may return NULL when the KeyData is invalid.
For such case, we should go to error handling path rather than assert

Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>