pgsql: Ensure that 'disabling statistics collector' is logged in all

Log Message:
-----------
Ensure that 'disabling statistics collector' is logged in all failure
paths of pgstat_init. Responds to confusion exhibited by Christoph Haller.

Modified Files:
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pgsql/src/backend/postmaster:
pgstat.c (r1.85 -> r1.86)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.85&r2=1.86)