apply: avoid possible bogus pointer

When parsing "index" lines from a git-diff, we look for a space followed by the mode. If we don't have a space, then we set our pointer to the end-of-line. However, we don't double-check that our end-of-line pointer is valid (e.g., if we got a truncated diff input), which could lead to some wrap-around pointer arithmetic. In most cases this would probably get caught by our "40 < len" check later in the function, but to be on the safe side, let's just use strchrnul to treat end-of-string the same as end-of-line. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>