patch 9.1.1564: crash when opening popup to closing buffer

Problem:  Can open a popup window to a closing buffer, leading to the
          buffer remaining open in the window after it's soon unloaded,
          causing crashes.
Solution: Check b_locked_split when opening a popup window to an
          existing buffer (Sean Dewar).

closes: #17790

Signed-off-by: Sean Dewar <[email protected]>
Signed-off-by: Christian Brabandt <[email protected]>

Author: seandewar

src/errors.h

@@ -3742,3 +3742,7 @@ EXTERN char e_cannot_have_more_than_nr_diff_anchors[]
 EXTERN char e_failed_to_find_all_diff_anchors[]
 	INIT(= N_("E1550: Failed to find all diff anchors"));
 #endif
+#ifdef FEAT_PROP_POPUP
+EXTERN char e_cannot_open_a_popup_window_to_a_closing_buffer[]
+	INIT(= N_("E1551: Cannot open a popup window to a closing buffer"));
+#endif