brcmfmac: validate user provided data for memdump before copying

In patch "brcmfmac: add support for sysfs initiated coredump", a new
scenario of brcmf_debug_create_memdump was added in which the user of
the function might not necessarily provide prefix data. Hence the
function should not assume the data is always valid and should perform a
check before copying.
Reviewed-by: NArend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: NFranky Lin <franky.lin@broadcom.com>
Signed-off-by: NArend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/broadcom/brcm80211/brcmfmac/debug.c

@@ -40,7 +40,8 @@ int brcmf_debug_create_memdump(struct brcmf_bus *bus, const void *data,
 	if (!dump)
 		return -ENOMEM;
 
-	memcpy(dump, data, len);
+	if (data && len > 0)
+		memcpy(dump, data, len);
 	err = brcmf_bus_get_memdump(bus, dump + len, ramsize);
 	if (err) {
 		vfree(dump);