pageinspect: Fix failure with hash_bitmap_info() for partitioned indexes

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