Fix unsafe use of strtol() on a non-null-terminated Text datum.

jsonb_set() could produce wrong answers or incorrect error reports, or in
the worst case even crash, when trying to convert a path-array element into
an integer for use as an array subscript.  Per report from Vitaly Burovoy.
Back-patch to 9.5 where the faulty code was introduced (in commit
c6947010).

Michael Paquier