scanner: fix at_son()

scanner: fix at_son()

Current test for "at start of node" is broken for 2 reasons.

1/ it doesn't account for the node-indent chars that are stripped
   off by do_strip()
2/ it check the ->offset *after* a character has been extracted,
   it needs to check the offset from before, which is in ->prev_offset

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