lib: add a few DEBUGASSERT(data) to aid code analyzers

... where 'data' is assumed to always work.

Closes #14033

lib/conncache.c

@@ -662,6 +662,7 @@ static void connc_discard_conn(struct conncache *connc,
   struct Curl_easy *data = last_data? last_data : connc->closure_handle;
   bool done = FALSE;
 
+  DEBUGASSERT(data);
   DEBUGASSERT(connc);
   DEBUGASSERT(!conn->bundle);
 
@@ -906,6 +907,7 @@ static void connc_perform(struct conncache *connc)
   if(!e)
     return;
 
+  DEBUGASSERT(data);
   DEBUGASSERT(!connc->shutdowns.iter_locked);
   DEBUGF(infof(data, "[CCACHE] perform, %zu connections being shutdown",
                Curl_llist_count(&connc->shutdowns.conn_list)));

lib/multi.c

@@ -1010,8 +1010,9 @@ void Curl_detach_connection(struct Curl_easy *data)
  * This is the only function that should assign data->conn
  */
 void Curl_attach_connection(struct Curl_easy *data,
-                             struct connectdata *conn)
+                            struct connectdata *conn)
 {
+  DEBUGASSERT(data);
   DEBUGASSERT(!data->conn);
   DEBUGASSERT(conn);
   data->conn = conn;