Use calloc to allocate new rds_index

The functions in the module like (rds_report_index_get_request) assume
zeroed memory.

Addressing SEGFAULT. (Stacktraces are from master, but segfault is
present in maint-1.0 branch as well).

  in rds_report_request_index_get_id (s=0x8282828282828282) at rds_index.c:62
  in ds_rds_session_select_report_request (session=0x73d7f0, report_request_id=0x0) at ds_rds_session.c:153
  in app_ds_rds_split (action=0x7fffffffd950) at oscap-ds.c:402
  in oscap_module_call (action=0x7fffffffd950) at oscap-tool.c:260
  in oscap_module_process (module=0x615ca0 <DS_RDS_SPLIT_MODULE>, argc=5, argv=0x7fffffffdbc8) at oscap-tool.c:345
  in main (argc=5, argv=0x7fffffffdbc8) at oscap.c:80

Conditional jump or move depends on uninitialised value(s)
  at 0x4C67564: ds_rds_session_select_report_request (ds_rds_session.c:150)
  by 0x409151: app_ds_rds_split (oscap-ds.c:402)
  by 0x407F88: oscap_module_call (oscap-tool.c:260)
  by 0x408405: oscap_module_process (oscap-tool.c:345)
  by 0x406E16: main (oscap.c:80)