efi_loader: avoid using unitialized values as console size

If a request for the console size would be answered with a response
with less then three values, uninitialized stack memory would be
copied to the number of rows and columns of the terminal.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>