git-annotate: fix -S on graft file with comments.

The graft file can contain comment lines and read_graft_line can
return NULL for such an input, which should be skipped by the
reader.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-blame.c

@@ -1407,7 +1407,8 @@ static int read_ancestry(const char *graft_file)
 		/* The format is just "Commit Parent1 Parent2 ...\n" */
 		int len = strlen(buf);
 		struct commit_graft *graft = read_graft_line(buf, len);
-		register_commit_graft(graft, 0);
+		if (graft)
+			register_commit_graft(graft, 0);
 	}
 	fclose(fp);
 	return 0;