bdiff: handle the possibility of an integer overflow when allocating

Differential Revision: https://phab.mercurial-scm.org/D1877

mercurial/bdiff.c

@@ -41,7 +41,7 b' int bdiff_splitlines(const char *a, ssiz'
 	if (p == plast)
 		i++;
 
-	*lr = l = (struct bdiff_line *)malloc(sizeof(struct bdiff_line) * i);
+	*lr = l = (struct bdiff_line *)calloc(i, sizeof(struct bdiff_line));
 	if (!l)
 		return -1;