Change -1 initial length values to 0
Change -1 initial length values to 0

Tree node length used to be type int, but is now size_t.
-1 (meaning 'incomplete') messed up printing of parse tree building
in progress (as seen with VM_PRINT_TREE)

ChrisHixon/chpeg,ChrisHixon/chpeg,ChrisHixon/chpeg,ChrisHixon/chpeg,ChrisHixon/chpeg