pwm: Fix possible ZERO_SIZE_PTR pointer dereferencing error.

Since we cannot make sure the 'chip->npwm' will always be none zero here,
and then if either equal to zero, the kzalloc() will return ZERO_SIZE_PTR,
which equals to ((void *)16).

So this patch fix this with just doing the zero check before calling kzalloc().
Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/core.c

@@ -236,7 +236,7 @@ int pwmchip_add(struct pwm_chip *chip)
 	int ret;
 
 	if (!chip || !chip->dev || !chip->ops || !chip->ops->config ||
-	    !chip->ops->enable || !chip->ops->disable)
+	    !chip->ops->enable || !chip->ops->disable || !chip->npwm)
 		return -EINVAL;
 
 	mutex_lock(&pwm_lock);