soc/amd/common/vboot: Verify the size of the transfer buffer

This will verify that signed verstage binaries and the bootblock code
executing agree on the transfer buffer struct size.

BUG=b:213828947
TEST=Boot guybrush to OS

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I597e38fe0a37416ffd3bc01fd974fa8f6610a88c
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61187
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kangheui Won <khwon@chromium.org>

src/soc/amd/common/vboot/vboot_bootblock.c

@@ -12,7 +12,7 @@
 
 static int transfer_buffer_valid(const struct transfer_info_struct *ptr)
 {
-	if (ptr->magic_val == TRANSFER_MAGIC_VAL)
+	if (ptr->magic_val == TRANSFER_MAGIC_VAL && ptr->struct_bytes == sizeof(*ptr))
 		return 1;
 	else
 		return 0;