Fix breakage of get_ps_display() in the PS_USE_NONE case.

Fix breakage of get_ps_display() in the PS_USE_NONE case.

Commit 8c6d30f21 caused this function to fail to set *displen
in the PS_USE_NONE code path.  If the variable's previous value
had been negative, that'd lead to a memory clobber at some call
sites.  We'd managed not to notice due to very thin test coverage
of such configurations, but this appears to explain buildfarm member
lorikeet's recent struggles.

Credit to Andrew Dunstan for spotting the problem.  Back-patch
to v13 where the bug was introduced.

Discussion: https://postgr.es/m/136102.1648320427@sss.pgh.pa.us