Danaj/unsafe decoding

Fix for issue 14: "Unsafe decoding creates infinite loop".

Version 0.26 on CPAN is missing both position checks.  The commit a year ago for issue 8 added one of them.  This adds the other.

I did not add anything to the tests.  Issue 14 has a simple test shown.

These changes should help with RT 27574 for Convert::PEM.