storage/flash_map: Return -ENODEV from flash_area_open

The commit adds check, to flash_area_open, whether there is any
device driver attached and returns -ENODEV if there isn't any.
This works around a problem where flash_area_open succeeds but
consecutive read/write causes crash.
It is enough to check the condition, and return error, here as
the flash_area_open has to precede, and be checked for success,
any read/write operations.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

subsys/storage/flash_map/flash_map.c

@@ -37,7 +37,12 @@ int flash_area_open(uint8_t id, const struct flash_area **fap)
 		return -ENOENT;
 	}
 
+	if (device_get_binding(area->fa_dev_name) == NULL) {
+		return -ENODEV;
+	}
+
 	*fap = area;
+
 	return 0;
 }