Make PerlIO::encoding even more resilient to moving buffers

Make PerlIO::encoding even more resilient to moving buffers

Commit 667763bdbf was not good enough.

If the buffer passed to an encode method is reallocated, it may be
smaller than the size (bufsiz) stored inside the encoding layer.  So
we need to extend the buffer in that case and make sure the buffer
pointer is not pointing to freed memory.

The test as modified by this commit causes malloc errors on stderr
when I try it without the encoding.xs changes.