Finish backporting RFC esi fixes

* the decoder was still working with the raw esi
  instead of the real (=raw - padding)
* The de_interleaver too, was still working
  with max_symbols == extended_symbols
  instead of max_symbols == real_symbols

also bakported a very minor out-of-one in the precode_solver
that would probably not affect anything

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>