play.c: Check whether filter ->close() is NULL.

play.c: Check whether filter ->close() is NULL.

While audiod.c and filter.c test whether ->close is NULL
before they attempt to call the function, play.c calls ->close()
unconditionally. This does not matter because all filters provide the
close method. But it is documented in filter.h that ->close may be
NULL. To avoid future surprises and to be consistent with para_audiod
and para_filter, let's change play.c to check for NULL as well.