postgres-lambda-diff

Oversight in 7d81bdc8, which introduced a new routine in perl lacking a
return clause.  Per buildfarm member crake.

Backpatch down to 9.6 like its parent.

Reported-by: Andrew Dunstan
Discussion: https://postgr.es/m/16da29fa-d504-1380-7095-40de586dc038@2ndQuadrant.com
Backpatch-through: 9.6