Use SSize_t when extending the stack
(I am referring to what is usually known simply as The Stack.)
This partially fixes #119161.
By casting the argument to int, we can end up truncating/wrapping
it on 64-bit systems, so EXTEND(SP,
2147483648) translates into
EXTEND(SP, -1), which does not extend the stack at all. Then writing
to the stack in code like ()=1..
1000000000000 goes past the end of
allocated memory and crashes.
I can’t really write a test for this, since instead of crashing it
will use more memory than I have available (and then I’ll start for-
getting things).