transport: fix incorrect statement

Commit 236208a5ff ("transport: use a bitmask for the transport")
has an incorrect C statement in place of a return.
The code is working thanks to the previous condition never true.

The issue has been detected by clang scan-build in OpenOCD ACI
since the missing return can make the following statement
dereferencing a NULL pointer.

Fix it!

Change-Id: I3bbe04d99ad9b1288f55ba3c45e2e487aef9ae40
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 236208a5ff ("transport: use a bitmask for the transport")
Reviewed-on: https://review.openocd.org/c/openocd/+/8868
Tested-by: jenkins

src/transport/transport.c

@@ -258,7 +258,7 @@ struct transport *get_current_transport(void)
 const char *get_current_transport_name(void)
 {
 	if (!session || !is_transport_id_valid(session->id))
-		NULL;
+		return NULL;
 
 	return transport_full_name(session->id);
 }