Fix framework message position boundary splits
Fix framework message position boundary splits

The framework would take the length of the prefix in bytes, and then
search for a command either directly after the byte length _and_ plus
one for a command. Instead, ensure that the message is longer
than the prefix.

Flat/serenity,eLunate/serenity,acdenisSK/serenity,Roughsketch/serenity,zeyla/serenity.rs,zeyla/serenity,Lakelezz/serenity