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>

color.c

@@ -207,6 +207,9 @@ int color_parse_mem(const char *value, int value_len, char *dst)
 	struct color fg = { COLOR_UNSPECIFIED };
 	struct color bg = { COLOR_UNSPECIFIED };
 
+	if (!len)
+		return -1;
+
 	if (!strncasecmp(value, "reset", len)) {
 		xsnprintf(dst, end - dst, GIT_COLOR_RESET);
 		return 0;