pageinspect: Fix failure with hash_bitmap_info() for partitioned indexes
This function reads directly a page from a relation, relying on
index_open() to open the index to read from. Unfortunately, this would
crash when using partitioned indexes, as these can be opened with
index_open() but they have no physical pages.
Alexander has fixed the module, while I have written the test.
Author: Alexander Lakhin, Michael Paquier
Discussion: https://postgr.es/m/18246-
f4d9ff7cb3af77e6@postgresql.org
Backpatch-through: 12