In planner.c, avoid assuming that all PathTargets have sortgrouprefs.

The struct definition for PathTarget specifies that a NULL sortgrouprefs
pointer means no sortgroupref labels.  While it's likely that there
should always be at least one labeled column in the places that were
unconditionally fetching through the pointer, it seems wiser to adhere to
the data structure specification and test first.  Add a macro to make this
convenient.  Per experimentation with running the regression tests with a
very small parallelization threshold --- the crash I observed may well
represent a bug elsewhere, but still this coding was not very robust.

Report: <20756.1465834072@sss.pgh.pa.us>