Add defenses to btree and hash index AMs to do simple sanity checks

on every index page they read; in particular to catch the case of an
all-zero page, which PageHeaderIsValid allows to pass.  It turns out
hash already had this idea, but it was just Assert()ing things rather
than doing a straight error check, and the Asserts were partially
redundant with PageHeaderIsValid anyway.  Per recent failure example
from Jim Nasby.  (gist still needs the same treatment.)

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