Sanity check cstore limit/offset param values

Sanity check cstore limit/offset param values

Certain cstore calls (direct / json_query) that support limit/offset
params called with a non-string / non-numeric value

e.g. { "limit": null }

result in a cstore segfault as it tries to call atoi(NULL) under the
covers.  This patch prevents this by verifying that the limit/offset
values are actual strings or numbers (i.e. return a value from
jsonObjectGetString) and not JSON_NULL, etc.

Signed-off-by: Bill Erickson <berick@esilibrary.com>