read_and_strip_branch: fix typo'd address-of operator

read_and_strip_branch: fix typo'd address-of operator

When we are chomping newlines from the end of a strbuf, we
must check "sb.len != 0" before accessing "sb.buf[sb.len - 1]".
However, this code mistakenly checks "&sb.len", which is
always true (it is a part of an auto struct, so the address
is always non-zero). This could lead to us accessing memory
outside the strbuf when we read an empty file.

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