Fix crash when decoding a = at end of the data

In decoding a quoted printable mail. There is a buffer overflow
as we are always parsing two bytes instead of one.

See the full backtrace at:

https://gist.github.com/sixstone-qq/cb8099b66c2911e8aaf2

This patch avoid regressions to manage = at the end of the source.
Merge pull request #13 from Zentyal/ejhernandez/fix-crash-decoding-quoted-printable-no-regressions

Fix crash when decoding a = at end of the data