Ensure debug symbols aren't mistaken as hero list
A system ROM is constructed from these parts in order
<modules><chain 0 terminator>[hero][debugsyms]<FF's>[footer]<checksums>
This causes a problem for things looking for hero text if there's no hero list but is room for debug symbols because the debug symbol count will be mistaken for a string. Solve this by adding a word of FF's before the debug symbols to ensure that the first thing encountered after the module chain 0 terminator word is either blankness or hero list, and never a debug symbol table.