strbuf: use size_t for length in intermediate variables

strbuf: use size_t for length in intermediate variables

A few strbuf functions store the length of a strbuf in a
temporary variable. We should always use size_t for this, as
it's possible for a strbuf to exceed an "int" (e.g., a 2GB
string on a 64-bit system). This is unlikely in practice,
but we should try to behave sensibly on silly or malicious
input.

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