pgsql: Fix logic bug in 1632ea43682f

Fix logic bug in 1632ea43682f

I overlooked that one condition was logically inverted. The fix is a
little bit more involved than simply negating the condition, to make
the code easier to read.

Fix some outdated comments left by the same commit, while at it.

Author: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/YMRlmB3/lZw8YBH+(at)paquier(dot)xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/33c509956704e1d918077b51ccd954f2e201a8f5

Modified Files
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src/backend/replication/slot.c | 30 ++++++++++++++++--------------
1 file changed, 16 insertions(+), 14 deletions(-)