color.c: fix color_parse_mem() with value_len == 0

In this code we want to match the word "reset". If len is zero,
strncasecmp() will return zero and we incorrectly assume it's "reset" as
a result.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>