DO-CODEC related fixes

This pull request fixes the regression crash introduced by https://github.com/rebol/rebol/pull/156 which caused TEXT type decoding operation non-functional.
It also fixes the crash when encoding image using PNG codec ([CC#2040](http://issue.cc/r3/2040)).

Here is a link to Gist that includes all basic DO-CODEC tests for possible reveal of regression:
https://gist.github.com/cyphre/9254624