color: add overflow checks for parsing colors

color: add overflow checks for parsing colors

Our color parsing is designed to never exceed COLOR_MAXLEN
bytes. But the relationship between that hand-computed
number and the parsing code is not at all obvious, and we
merely hope that it has been computed correctly for all
cases.

Let's mark the expected "end" pointer for the destination
buffer and make sure that we do not exceed it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>