add an Assertion about a buffer overflow vulnerability in split_str

git-svn-id: svn://svn.icculus.org/fs2open@11037 387891d4-d844-0410-90c0-e4c51a9137d3
Karajorma missed an Opps :p

git-svn-id: svn://svn.icculus.org/fs2open@11038 387891d4-d844-0410-90c0-e4c51a9137d3
since we can replace variables in SCP_string, use that instead of an arbitrary char[256] buffer
also const-correctness in the constructor

git-svn-id: svn://svn.icculus.org/fs2open@11039 387891d4-d844-0410-90c0-e4c51a9137d3
subtitles can now be as long as messages

git-svn-id: svn://svn.icculus.org/fs2open@11040 387891d4-d844-0410-90c0-e4c51a9137d3
that variable isn't actually a buffer, so change the misleading name

git-svn-id: svn://svn.icculus.org/fs2open@11041 387891d4-d844-0410-90c0-e4c51a9137d3