pgsql: Fix core dump in jsonb #> operator, and add regression test case

Fix core dump in jsonb #> operator, and add regression test cases.

jsonb's #> operator segfaulted (dereferencing a null pointer) if the RHS
was a zero-length array, as reported in bug #11207 from Justin Van Winkle.
json's #> operator returns NULL in such cases, so for the moment let's
make jsonb act likewise.

Also add a bunch of regression test queries memorializing the -> and #>
operators' behavior for this and other corner cases.

There is a good argument for changing some of these behaviors, as they
are not very consistent with each other, and throwing an error isn't
necessarily a desirable behavior for operators that are likely to be
used in indexes. However, everybody can agree that a core dump is the
Wrong Thing, and we need test cases even if we decide to change their
expected output later.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/9bac66020db75871463bafdb394568bf946e8991

Modified Files
--------------
src/backend/utils/adt/jsonfuncs.c | 16 ++-
src/test/regress/expected/json.out | 185 +++++++++++++++++++++++++++++----
src/test/regress/expected/json_1.out | 185 +++++++++++++++++++++++++++++----
src/test/regress/expected/jsonb.out | 170 +++++++++++++++++++++++++-----
src/test/regress/expected/jsonb_1.out | 170 +++++++++++++++++++++++++-----
src/test/regress/sql/json.sql | 49 +++++++--
src/test/regress/sql/jsonb.sql | 55 +++++++---
7 files changed, 711 insertions(+), 119 deletions(-)