ctrl_cmd_parse: Make coverity FORWARD_NULL happy

The 'strtok_r' function requires passing a NULL as the first parameter
on subsequent calls in order to ensure the code picks up where it left
off on a previous call.  However, Coverity doesn't quite realize this
and points out that if a NULL was passed in as the third argument it
would result in a possible NULL deref because the strtok_r function will
assign the third argument to the first in the call is NULL.

Change-Id: I7a9d08d0d4eae76a5207d285e32a25d1b384a57f
Fixes: Coverity CID 135186

src/ctrl/control_cmd.c

@@ -299,6 +299,7 @@ struct ctrl_cmd *ctrl_cmd_parse(void *ctx, struct msgb *msg)
 	msgb_put_u8(msg, 0);
 	str = (char *) msg->l2h;
 
+	OSMO_ASSERT(str);
 	tmp = strtok_r(str, " ",  &saveptr);
 	if (!tmp) {
 		cmd->type = CTRL_TYPE_ERROR;