fix up EXTEND() callers

fix up EXTEND() callers

The previous commit made it clear that the N argument to EXTEND()
is supposed to be signed, in particular SSize_t, and now typically
triggers compiler warnings where this isn't the case.

This commit fixes the various places in core that passed the wrong sort of
N to EXTEND(). The fixes are in three broad categories.

First, where sensible, I've changed the relevant var to be SSize_t.

Second, where its expected that N could never be large enough to wrap,
I've just added an assert and a cast.

Finally, I've added extra code to detect whether the cast could
wrap/truncate, and if so set N to -1, which will trigger a panic in
stack_grow().

This also fixes
    [perl #125937] 'x' operator on list causes segfault with possible
    stack corruption