regulator: Fix possible nullpointer dereference in regulator_enable()

In the case where _regulator_enable returns an error it was not checked
if a supplying regulator exists before trying to disable it, leading
to a null pointer-dereference if no supplying regulator existed.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/core.c

@@ -1425,7 +1425,7 @@ int regulator_enable(struct regulator *regulator)
 	ret = _regulator_enable(rdev);
 	mutex_unlock(&rdev->mutex);
 
-	if (ret != 0)
+	if (ret != 0 && rdev->supply)
 		regulator_disable(rdev->supply);
 
 	return ret;