cifs: Fix oops in session setup code for null user mounts

For null user mounts, do not invoke string length function
during session setup.

Cc: <[e-mail]
Reported-and-Tested-by: Chris Clayton <[e-mail]>
Acked-by: Jeff Layton <[e-mail]>
Signed-off-by: Shirish Pargaonkar <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/sess.c

···
 	/* copy user */
 	/* BB what about null user mounts - check that we do this BB */
 	/* copy user */
-	if (ses->user_name != NULL)
+	if (ses->user_name != NULL) {
 		strncpy(bcc_ptr, ses->user_name, MAX_USERNAME_SIZE);
+		bcc_ptr += strnlen(ses->user_name, MAX_USERNAME_SIZE);
+	}
 	/* else null user mount */
-
-	bcc_ptr += strnlen(ses->user_name, MAX_USERNAME_SIZE);
 	*bcc_ptr = 0;
 	bcc_ptr++; /* account for null termination */
 
 	/* copy domain */
-
 	if (ses->domainName != NULL) {
 		strncpy(bcc_ptr, ses->domainName, 256);
 		bcc_ptr += strnlen(ses->domainName, 256);