efi/libstub: Avoid returning uninitialized data from setup_graphics()

Currently, setup_graphics() ignores the return value of efi_setup_gop(). As
AllocatePool() does not zero out memory, the screen information table will
contain uninitialized data in this case.

We should free the screen information table if efi_setup_gop() returns an
error code.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Link: https://lore.kernel.org/r/20200426194946.112768-1-xypron.glpk@gmx.de


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>