patch 8.2.0834: :drop command in terminal popup causes problems

Problem:    :drop command in terminal popup causes problems.
Solution:   Check for using a popup window. (closes #6151)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 9c55de3..777f149 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -6349,6 +6349,9 @@
     buf_T	*buf;
     tabpage_T	*tp;
 
+    if (ERROR_IF_POPUP_WINDOW || ERROR_IF_TERM_POPUP_WINDOW)
+	return;
+
     /*
      * Check if the first argument is already being edited in a window.  If
      * so, jump to that window.