pgsql: During index build, check and elog (not just Assert) for broken

During index build, check and elog (not just Assert) for broken HOT chain.

The recently-fixed bug in WAL replay could result in not finding a parent
tuple for a heap-only tuple.  The existing code would either Assert or
generate an invalid index entry, neither of which is desirable.  Throw a
regular error instead.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/c3701254b58d2323dfa399d3ed62551e2c738c48

Modified Files
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src/backend/catalog/index.c |   11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)