pretty: fix integer overflow in wrapping format

pretty: fix integer overflow in wrapping format

The `%w(width,indent1,indent2)` formatting directive can be used to
rewrap text to a specific width and is designed after git-shortlog(1)'s
`-w` parameter. While the three parameters are all stored as `size_t`
internally, `strbuf_add_wrapped_text()` accepts integers as input. As a
result, the casted integers may overflow. As these now-negative integers
are later on passed to `strbuf_addchars()`, we will ultimately run into
implementation-defined behaviour due to casting a negative number back
to `size_t` again. On my platform, this results in trying to allocate
9000 petabyte of memory.

Fix this overflow by using `cast_size_t_to_int()` so that we reject
inputs that cannot be represented as an integer.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>