config.c: avoid integer truncation in `copy_or_rename_section_in_file()`

config.c: avoid integer truncation in `copy_or_rename_section_in_file()`

There are a couple of spots within `copy_or_rename_section_in_file()`
that incorrectly use an `int` to track an offset within a string, which
may truncate or wrap around to a negative value.

Historically it was impossible to have a line longer than 1024 bytes
anyway, since we used fgets() with a fixed-size buffer of exactly that
length. But the recent change to use a strbuf permits us to read lines
of arbitrary length, so it's possible for a malicious input to cause us
to overflow past INT_MAX and do an out-of-bounds array read.

Practically speaking, however, this should never happen, since it
requires 2GB section names or values, which are unrealistic in
non-malicious circumstances.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>