Fix crash when moving container to marked workspace

Was a small typo.

This also has the (positive) side-effect of allowing to move all the
content of a marked workspace next to the target container, see added
tests.

src/con.c

@@ -1305,7 +1305,7 @@ bool con_move_to_mark(Con *con, const char *mark) {
         return true;
     }
 
-    if (con->type == CT_WORKSPACE) {
+    if (target->type == CT_WORKSPACE) {
         DLOG("target container is a workspace, simply moving the container there.\n");
         con_move_to_workspace(con, target, true, false, false);
         return true;