patch 8.2.1807: can use :help in a terminal popup window

Problem:    Can use :help in a terminal popup window.
Solution:   Give an error. (closes #7088)
diff --git a/src/help.c b/src/help.c
index 4f2b6b6..4d59ab9 100644
--- a/src/help.c
+++ b/src/help.c
@@ -39,6 +39,9 @@
     int		old_KeyTyped = KeyTyped;
 #endif
 
+    if (ERROR_IF_ANY_POPUP_WINDOW)
+	return;
+
     if (eap != NULL)
     {
 	// A ":help" command ends at the first LF, or at a '|' that is