Do not access indclass through Form_pg_index

Do not access indclass through Form_pg_index

Normally, accessing variable-length members of catalog structures past
the first one doesn't work at all.  Here, it happened to work because
indnatts was checked to be 1, and so the defined FormData_pg_index
layout, using int2vector[1] and oidvector[1] for variable-length
arrays, happened to match the actual memory layout.  But it's a very
fragile assumption, and it's not in a performance-critical path, so
code it properly using heap_getattr() instead.

bug analysis by Tom Lane