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 <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/maps/physmap-core.c

···
 	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;