Perl_do_vecget(): change offset arg to STRLEN type

Perl_do_vecget(): change offset arg to STRLEN type

... and fix up its caller, pp_vec().

This is part of a fix for RT #130915.

pp_vec() is responsible for extracting out the offset and size from SVs on
the stack, and then calling do_vecget() with those values. (Sometimes the
call is done indirectly by storing the offset in the LvTARGOFF() field of
a SVt_PVLV, then later Perl_magic_getvec() passes the LvTARGOFF() value to
do_vecget().)

Now SvCUR, SvLEN and LvTARGOFF are all of type STRLEN (a.k.a Size_t),
while the offset arg of do_vecget() is of type SSize_t (i.e. there's a
signed/unsigned mismatch). It makes more sense to make the arg of type
STRLEN. So that is what this commit does.

At the same time this commit fixes up pp_vec() to handle all the
possibilities where the offset value can't fit into a STRLEN, returning 0
or croaking accordingly, so that do_vecget() is never called with a
truncated or wrapped offset.

The next commit will fix up the internals of do_vecget() and do_vecset(),
which have to worry about offset*(2^n) wrapping or being > SvCUR().

This commit is based on an earlier proposed fix by Aaron Crane.