gitk: Fix error message when clicking on a connecting line

When clicking on the line that connects two commit nodes, gitk
would bring up an error dialog saying "can't read "cflist_top":
no such variable".

This fixes a regression that was introduced with b967135 ("gitk:
Synchronize highlighting in file view when scrolling diff").

Signed-off-by: Stefan Haller <stefan@haller-berlin.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -7970,6 +7970,8 @@ proc changediffdisp {} {
 proc highlightfile {cline} {
     global cflist cflist_top
 
+    if {![info exists cflist_top]} return
+
     $cflist tag remove highlight $cflist_top.0 "$cflist_top.0 lineend"
     $cflist tag add highlight $cline.0 "$cline.0 lineend"
     $cflist see $cline.0