pgsql: Fix a thinko introduced into CountActiveBackends by a recent

Log Message:
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Fix a thinko introduced into CountActiveBackends by a recent patch:
we should ignore NULL array entries, not non-NULL ones.  This had the
effect of disabling commit_delay, and could have caused a crash in the
rare race condition the patch was intended to fix.

Bug report and diagnosis by Jeff Janes, in bug #4952.

Tags:
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REL8_4_STABLE

Modified Files:
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    pgsql/src/backend/storage/ipc:
        procarray.c (r1.50 -> r1.50.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.50&r2=1.50.2.1)