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

src/backend/utils/adt/jsonfuncs.c

@@ -3873,7 +3873,7 @@ setPathArray(JsonbIterator **it, Datum *path_elems, bool *path_nulls,
 	/* pick correct index */
 	if (level < path_len && !path_nulls[level])
 	{
-		char	   *c = VARDATA_ANY(path_elems[level]);
+		char	   *c = TextDatumGetCString(path_elems[level]);
 		long		lindex;
 
 		errno = 0;