patch 8.1.1207: some compilers give warning messages

Problem:    Some compilers give warning messages.
Solution:   Initialize variables, change printf() argument. (Christian
            Brabandt, closes #4305)
diff --git a/src/window.c b/src/window.c
index df82d20..b132361 100644
--- a/src/window.c
+++ b/src/window.c
@@ -3981,7 +3981,7 @@
     void
 goto_tabpage(int n)
 {
-    tabpage_T	*tp;
+    tabpage_T	*tp = NULL;  // shut up compiler
     tabpage_T	*ttp;
     int		i;