Setting $_ to multiline glob in @INC filter

Setting $_ to multiline glob in @INC filter

This commit corrects the one remaining code path in
pp_ctl.c:S_run_user_filter that was not handling non-PVs.

If we have to truncate $_ and keep the remainder in a cache (because
it has more than one line), we need to make sure we don’t assume it is
a PV when it comes to truncating it.

$ ./perl -Ilib -e '@INC = sub { sub { return 0 if $|--; $_ = *{"foo;\nbar"}; return 1 } }; do "foo"'
Assertion failed: (!isGV_with_GP(upstream)), function S_run_user_filter, file pp_ctl.c, line 5494.
Abort trap: 6