maint: fix an inconsequential memory leak in join

maint: fix an inconsequential memory leak in join

* src/join.c (join): Refactor the code that checks for misorder
at the tail of the files.  The most significant change here is
that freeline() is called thus silencing a valgrind warning about
a definite but inconsequential memory leak.
(freeline): Make more general by doing nothing when passed NULL,
and setting freed pointers to NULL.