efi/x86: Add missing NULL initialization in UGA draw protocol discovery

The UGA draw protocol discovery routine looks for a EFI handle that has
both the UGA draw protocol and the PCI I/O protocol installed. It checks
for the latter by calling handle_protocol() and pass it a PCI I/O
protocol pointer variable by reference, but fails to initialize it to
NULL, which means the non-NULL check later on in the code could produce
false positives, given that the return code of the handle_protocol() call
is ignored entirely. So add the missing initialization.

Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20180720014726.24031-6-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/eboot.c

@@ -359,6 +359,7 @@ setup_uga(struct screen_info *si, efi_guid_t *uga_proto, unsigned long size)
 		if (status != EFI_SUCCESS)
 			continue;
 
+		pciio = NULL;
 		efi_call_early(handle_protocol, handle, &pciio_proto, &pciio);
 
 		status = efi_call_proto(efi_uga_draw_protocol, get_mode, uga,