soc/intel/{cannonlake,skylake}: Fix null pointer dereference in klocwork

This patch fixes klocwork bug due to recent memmap.c
implementation where “Pointer 'dev' returned from call
to function 'dev_find_slot' at line 144 may be NULL.”

Change-Id: I4c74ca410d1a0ba48634ec9928a0d9d1cc20e27a
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://review.coreboot.org/21404
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/cannonlake/memmap.c

@@ -91,6 +91,8 @@ static uintptr_t calculate_dram_base(void)
 	size_t imr_size;
 
 	dev = dev_find_slot(0, PCI_DEVFN(SA_DEV_SLOT_IGD, 0));
+	if (!dev)
+		die("ERROR - device not found!");
 
 	/* Read TOLUD from Host Bridge offset */
 	dram_base = sa_get_tolud_base();