Fix: GPv3 layer renaming when name is unchanged

When old and new names of layers/groups are same, garbage string is
displaed. This is because `oldname` char pointer is referenced to
`node.name`. After function execution, `oldname` argument is freed in
`interface_handler` (see `after.rename_orig`), thus displays garbage
string.

@@ -864,6 +864,9 @@ static void namebutton_fn(bContext *C, void *tsep, char *oldname)
           /* The node already has the new name set. To properly rename the node, we need to first
            * store the new name, restore the old name in the node, and then call the rename
            * function. */
+          if (strcmp(node.name, oldname) == 0) {

@@ -867,2 +867,3 @@
+
           std::string new_name(node.name);
-          node.name = oldname;
+          BLI_strncpy(node.name, oldname, sizeof(oldname));