crash in DROP INDEX CONCURRENTLY

I just observed that if I run src/test/regress' "make
installcheck-parallel" and concurrently src/test/isolation's "make
installcheck", I very reproducibly get a segfault (in an assert-enabled
build).  Apparently the problem is that somebody is trying to transfer
predicate locks to a relation, but the index' relcache entry is pfreed.

I think the fix is just to move the heap_open calls some lines upwards, as in
the attached patch.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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