parsergen: be careful shifting TK_error

parsergen: be careful shifting TK_error

shift() behaved a little differently when p.tos == 0,
and if the stack is completely empty, there is little
point trying to shift TK_error as there is no state
to work with.

So rearrange the loop slightly.

Signed-off-by: NeilBrown <neil@brown.name>