don't return uninitialized memory from bdiff.blocks()

bdiff.blocks() returns a dummy match at the end of both files; the
length of that chunk is never set, so it will sometimes contain random
heap garbage. There are apparently workarounds for this elsewhere:

  # bdiff sometimes gives huge matches past eof, this check eats them,

mercurial/bdiff.c

@@ -251,8 +251,8 b' static struct hunklist diff(struct line '
 	if (pos && l.base && t) {
 		/* generate the matching block list */
 		recurse(a, b, pos, 0, an, 0, bn, &l);
-		l.head->a1 = an;
-		l.head->b1 = bn;
+		l.head->a1 = l.head->a2 = an;
+		l.head->b1 = l.head->b2 = bn;
 		l.head++;
 	}