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