pgsql: Only adjust negative indexes in json_get up to the length of the

Only adjust negative indexes in json_get up to the length of the path.

The previous code resulted in memory access beyond the path bounds. The
cure is to move it into a code branch that checks the value of lex_level
is within the correct bounds.

Bug reported and diagnosed by Piotr Stefaniak.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/40a50a17b905dae233ddb8bb36b7deff9e3abb16

Modified Files
--------------
src/backend/utils/adt/jsonfuncs.c | 26 +++++++++++++-------------
1 file changed, 13 insertions(+), 13 deletions(-)

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