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>