layout.c: Validate _layout_entry_by_name() arguments before use

It may be the case that a layout could not be derived which
would result in layout logic being fed a NULL pointer. Validate
this case and be defensive to validate the name argument as well.

BUG=b:247055486
TEST=builds

Change-Id: I2a19c0e586f8575b8b3c2c02b5afad312efacfc9
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/67722
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

layout.c

@@ -58,6 +58,8 @@ static struct romentry *_layout_entry_by_name(
 		const struct flashrom_layout *const layout, const char *name)
 {
 	struct romentry *entry = NULL;
+	if (!layout || !name)
+		return NULL;
 	while ((entry = mutable_layout_next(layout, entry))) {
 		if (!strcmp(entry->name, name))
 			return entry;