When blk_full() handles an .It line in .Bl -column and indirectly

When blk_full() handles an .It line in .Bl -column and indirectly
calls phrase_ta() to handle a .Ta child macro, advance the body
pointer accordingly, such that a subsequent tab character rewinds
the right body block and doesn't fail an assertion.  That happened
when there was nothing between the .Ta and the tab character.
Bug reported by tim@ some time ago.