pgsql: Make GetSlotInvalidationCause() return RS_INVAL_NONE on unexpect

Make GetSlotInvalidationCause() return RS_INVAL_NONE on unexpected input

943f7ae1c869 has changed GetSlotInvalidationCause() so as it would
return the last element of SlotInvalidationCauses[] when an incorrect
conflict reason name is given by a caller, but this should return
RS_INVAL_NONE in such cases, even if such a state should never be
reached in practice.

Per gripe from Peter Smith.

Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/CAHut+PtsrSWxczpGkSaSVtJo+BXrvJ3Hwp5gES14bbL-G+HL7A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/efa70c15c742511195e2ee6f0aef94d0797daf80

Modified Files
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src/backend/replication/slot.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)