patch 9.2.0487: viminfo: possible signed int overflow in register array

patch 9.2.0487: viminfo: possible signed int overflow in register array

Problem:  viminfo: possible signed int overflow in register array growth
Solution: Cast to size_t (Yasuhiro Matsumoto)

The expression `limit * 2 * sizeof(string_T)` in read_viminfo_register()
multiplies in int and overflows once limit exceeds INT_MAX/2. Cast to
size_t first so the size computation stays unsigned. Defensive only;
reaching this path requires registers consuming many gigabytes.

closes: #20207

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>