Use size_t type for arguments passed to iconv()

Use size_t type for arguments passed to iconv()

On system where sizeof(size_t) != sizeof(uint32_t) iconv() was failing
and even segfaulting. Use the correct type and remove the now unneded
casts from the iconv() invocations.

This makes RCOmage finally work in linux on 64 bit systems as well.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>