net: phy: mdio-mux: Check return value of mdiobus_alloc()

commit 20b08e1a upstream.

mdiobus_alloc() might return NULL, but its return value is not
checked in mdio_mux_init(). This could potentially lead to a NULL
pointer dereference. Fix it by checking the return value

Fixes: 0ca2997d ("netdev/of/phy: Add MDIO bus multiplexer support.")
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/net/phy/mdio-mux.c

@@ -148,9 +148,14 @@ int mdio_mux_init(struct device *dev,
 		}
 		cb->bus_number = v;
 		cb->parent = pb;
+
 		cb->mii_bus = mdiobus_alloc();
+		if (!cb->mii_bus) {
+			ret_val = -ENOMEM;
+			of_node_put(child_bus_node);
+			break;
+		}
 		cb->mii_bus->priv = cb;
-
 		cb->mii_bus->irq = cb->phy_irq;
 		cb->mii_bus->name = "mdio_mux";
 		snprintf(cb->mii_bus->id, MII_BUS_ID_SIZE, "%x.%x",