[perl #115440] Fix various leaks with fatal FETCH

[perl #115440] Fix various leaks with fatal FETCH

Various pieces of code were creating an SV and then assigning to it
from a value that might be magical.  If the source scalar is magical,
it could die when magic is called, leaking the scalar that would have
been assigned to.

So we call get-magic before creating the new scalar, and then use a
non-magical assignment.

Also, anonhash and anonlist were doing nothing to protect the aggre-
gate if an argument should die on FETCH, resulting in a leak.