libpq: Prevent some overflows of int/size_t

libpq: Prevent some overflows of int/size_t

Several functions could overflow their size calculations, when presented
with very large inputs from remote and/or untrusted locations, and then
allocate buffers that were too small to hold the intended contents.

Switch from int to size_t where appropriate, and check for overflow
conditions when the inputs could have plausibly originated outside of
the libpq trust boundary. (Overflows from within the trust boundary are
still possible, but these will be fixed separately.) A version of
add_size() is ported from the backend to assist with code that performs
more complicated concatenation.

Reported-by: Aleksey Solovev (Positive Technologies)
Reviewed-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Security: CVE-2025-12818
Backpatch-through: 13