patch 7.4.1704

Problem:    Using freed memory with "wincmd p". (Dominique Pelle)
Solution:   Also clear "prevwin" in other tab pages.

src/version.c

@@ -748,6 +748,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1704,
 /**/
     1703,
 /**/

src/window.c

@@ -340,7 +340,7 @@ newwindow:
 /* cursor to last accessed (previous) window */
     case 'p':
     case Ctrl_P:
-		if (prevwin == NULL)
+		if (!win_valid(prevwin))
 		    beep_flush();
 		else
 		    win_goto(prevwin);
@@ -4577,8 +4577,15 @@ win_free(
     unref_var_dict(wp->w_vars);
 #endif
 
+    {
+	tabpage_T	*ttp;
+
 	if (prevwin == wp)
 	    prevwin = NULL;
+	for (ttp = first_tabpage; ttp != NULL; ttp = ttp->tp_next)
+	    if (ttp->tp_prevwin == wp)
+		ttp->tp_prevwin = NULL;
+    }
     win_free_lsize(wp);
 
     for (i = 0; i < wp->w_tagstacklen; ++i)