fix calloc(0, ...) issue

mercurial/bdiff.c

@@ -245,7 +245,7 b' static struct hunklist diff(struct line '
 
 	/* allocate and fill arrays */
 	t = equatelines(a, an, b, bn);
-	pos = (struct pos *)calloc(bn, sizeof(struct pos));
+	pos = (struct pos *)calloc(bn ? bn : 1, sizeof(struct pos));
 	/* we can't have more matches than lines in the shorter file */
 	l.head = l.base = (struct hunk *)malloc(sizeof(struct hunk) *
 	                                        ((an<bn ? an:bn) + 1));