pgsql: Don't read fields of a misaligned ExpandedObjectHeader orAnyArr

Don't read fields of a misaligned ExpandedObjectHeader or AnyArrayType.

UBSan complains about this.  Instead, cast to a suitable type requiring
only 4-byte alignment.  DatumGetAnyArrayP() already assumes one can cast
between AnyArrayType and ArrayType, so this doesn't introduce a new
assumption.  Back-patch to 9.5, where AnyArrayType was introduced.

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/20190629210334.GA1244217@rfd.leadboat.com

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b664b187d7ea6546ef4e0f32167a93c6a74dca08

Modified Files
--------------
src/backend/utils/adt/arrayfuncs.c |  2 +-
src/include/utils/array.h          | 19 +++++++++++++------
src/include/utils/arrayaccess.h    |  4 ++--
src/include/utils/expandeddatum.h  |  2 +-
4 files changed, 17 insertions(+), 10 deletions(-)