mtd: physmap-core: Fix NULL pointer dereferencing in of_select_probe_type()

Coverity complains of a possible NULL dereference:

in of_select_probe_type():
1. returned_null: of_match_device() returns NULL.
2. var_assigned: match = NULL return value from of_match_device()
309	match = of_match_device(of_flash_match, &dev->dev);

3.dereference: Dereferencing the NULL pointer match.
310	probe_type = match->data;
Signed-off-by: Zeng Jingxiang <linuszeng@tencent.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220727060302.1560325-1-zengjx95@gmail.com

drivers/mtd/maps/physmap-core.c

@@ -300,6 +300,9 @@ static const char *of_select_probe_type(struct platform_device *dev)
 	const char *probe_type;
 
 	match = of_match_device(of_flash_match, &dev->dev);
+	if (!match)
+		return NULL;
+
 	probe_type = match->data;
 	if (probe_type)
 		return probe_type;