Revert "pdu.c: Fix length check in write_{prefix,char}"
Revert "pdu.c: Fix length check in write_{prefix,char}"

This reverts commit 34b8484a0c9d13db4c6086c880e40484ef9177dd.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap