combine-diff: fix loop index underflow

If both la and context are zero at the start of the loop, la wraps around
and we end up reading from memory far away.  Skip the loop in that case
instead.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: René Scharfe

combine-diff.c

@@ -414,7 +414,7 @@ static int make_hunks(struct sline *sline, unsigned long cnt,
 						     hunk_begin, j);
 				la = (la + context < cnt + 1) ?
 					(la + context) : cnt + 1;
-				while (j <= --la) {
+				while (la && j <= --la) {
 					if (sline[la].flag & mark) {
 						contin = 1;
 						break;