When the head of a list item is extended with a partial explicit

When the head of a list item is extended with a partial explicit
macro (for example .Xo) and never closed again, the item ends up
without a body block.  This can even happen for list types that
usually don't have heads in the first place.  So even in this
case, check for the existence of the body before accessing it.
NULL pointer access found by jsg@ with afl.