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
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/6d10f4e9d7f0051afb60c42409f2fe61cf4da348

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

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