git-gui: fix exception when trying to stage with empty file list

If there is nothing to stage, there is nothing to stage. Let's not try
to, even if the file list contains nothing at all.

This fixes #1075

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

git-gui/git-gui.sh

@@ -2516,7 +2516,9 @@ proc toggle_or_diff {mode w args} {
 		if {$last_clicked ne {}} {
 			set lno [lindex $last_clicked 1]
 		} else {
-			if {[llength $file_lists($w)] == 0} {
+			if {![info exists file_lists]
+				|| ![info exists file_lists($w)]
+				|| [llength $file_lists($w)] == 0} {
 				set last_clicked {}
 				return
 			}