su_uplevel: populate lower stack frames properly

su_uplevel: populate lower stack frames properly

When creating a temporary new argument stack, su_uplevel() copies most of
the old stack to a new one. This isn't really needed, as the new stack
will be abandoned before it ever pops back to that level.  But it *is*
needed when debugging code prints out the stack, as in for example,
'perl -Dstv'.

However, the code didn't actually copy the old stack: it copied garbage
instead, since it was using PL_curstack rather than AvARRAY(PL_curstack)
as the address of of the old stack. Which was causing 'perl -Dstv' to
SEGV.

This commit fixes that.