Reject input sizes that overflow the execute buffer size

allocsize is in_size * sizeof(*in) + FN_NAME_LENGTH with no overflow
check, and the buffer is then unconditionally written for
FN_NAME_LENGTH bytes. in_size = 0xFFFFFFFFFFFFFFFF makes allocsize wrap
to 24, which fails the "> 512" heap test, so the buffer comes from
_alloca(24) and RtlZeroMemory(p, 32) runs 8 bytes past it; lstrcpynA
can write the full 32. Four wrapping values reach the same place.

Guard both pawnio_execute_nt and pawnio_execute_async_nt next to the
existing name-length check.

Verified against the built DLL with a bogus handle, since the check
runs before the handle is touched: all six wrapping sizes now return
STATUS_INVALID_PARAMETER, an ordinary size still gets through to
STATUS_INVALID_HANDLE, and the same test against the previous build
died with 0xC0000409, STATUS_STACK_BUFFER_OVERRUN.