regmap: Fix possible ZERO_SIZE_PTR pointer dereferencing error.

Since we cannot make sure the 'len = pair_size * num_regs' will always
be none zero from the users, and then if 'num_regs' equals to zero by
mistake or other reasons, the kzalloc() will return ZERO_SIZE_PTR, which
equals to ((void *)16).

So this patch fix this with just doing the 'len' zero check before calling
kzalloc().

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/base/regmap/regmap.c

@@ -1615,6 +1615,9 @@ static int _regmap_raw_multi_reg_write(struct regmap *map,
 	size_t pair_size = reg_bytes + pad_bytes + val_bytes;
 	size_t len = pair_size * num_regs;
 
+	if (!len)
+		return -EINVAL;
+
 	buf = kzalloc(len, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;