[PATCH-resent] gitk: fix in procedure drawcommits

This patch indroduces a check before unsetting an array element.

Without this, gitk may complain with

	can't unset "prevlines(...)": no such element in array

when scrolling happens.

Signed-off-by: Michele Ballabio <redacted>
---

There's an error that seems to occur in gitk only on
mutt's imported repo, but I don't know why. This is
hopefully the right fix.

An example of this error:

can't unset "prevlines(a3b4383d69e0754346578c85ba8ff7c05bd88705)": no such element in array
can't unset "prevlines(a3b4383d69e0754346578c85ba8ff7c05bd88705)": no such element in array
    while executing
"unset prevlines($lid)"
    (procedure "drawcommits" line 39)
    invoked from within
"drawcommits $row $endrow"
    (procedure "drawfrac" line 10)
    invoked from within
"drawfrac $f0 $f1"
    (procedure "scrollcanv" line 3)
    invoked from within
"scrollcanv .tf.histframe.csb 0.00672513 0.0087015"

 gitk |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/gitk b/gitk
index 300fdce..527b716 100755
--- a/gitk
+++ b/gitk

@@ -3697,7 +3697,9 @@ proc drawcommits {row {endrow {}}} {
 
 	if {[info exists lineends($r)]} {
 	    foreach lid $lineends($r) {
-		unset prevlines($lid)
+		if {[info exists prevlines($lid)]} {
+		    unset prevlines($lid)
+		}
 	    }
 	}
 	set rowids [lindex $rowidlist $r]

-- 
1.5.3