builtin-grep: terminate correctly at EOF

It barfed and segfaulted with an incomplete line.

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

builtin-grep.c

@@ -264,6 +264,8 @@ static int grep_buffer(struct grep_opt *opt, const char *name,
 		}
 		*eol = ch;
 		bol = eol + 1;
+		if (!left)
+			break;
 		left--;
 		lno++;
 	}