Reduce memory fragmentation when running encrypted Perl scripts

The use of mortal SVs in FilterCrypto_FilterDecrypt() could lead to
excessive fragmentation of the heap, causing apparently high memory usage
even though all allocated memory was freed and little was actually in use
at any given time.

As an example, a simple "Hello, world." program with a 50,000 line POD
section after it normally uses about 4MB (working set) on my Windows 7
machine, with peak working set also 4MB, and virtual size (the size of the
virtual address space, i.e. how much memory has been addressed by the
process) peaking at about 51MB. However, when encrypted, the same program
used about 10MB, but peaked at 97MB, and had addressed as much as 197MB!

This patch is based on one from Kevin Groeneveld <kgroeneveld@gmail.com>,
who diagnosed and reported the problem in cpan #95021.