brcmfmac: add parameter check in brcmf_c_mkiovar()

This patch assures data is only appended in output buffer when
data is not NULL and datalen is not zero.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmfmac/dhd_common.c

@@ -80,8 +80,10 @@ brcmf_c_mkiovar(char *name, char *data, uint datalen, char *buf, uint buflen)
 	strncpy(buf, name, buflen);
 
 	/* append data onto the end of the name string */
-	memcpy(&buf[len], data, datalen);
-	len += datalen;
+	if (data && datalen) {
+		memcpy(&buf[len], data, datalen);
+		len += datalen;
+	}
 
 	return len;
 }