AvfDecoder: fix several Coverity detected errors

* 1026172 Uninitialized scalar variable (fill).
* 1026173 Uninitialized scalar variable (total).
* 1026164 Division or modulo by zero - this looks like a false positive
          because m_inited will be false if m_sampleFmt is FORMAT_NONE
          which is the only way the division by 0 can happen.