Fixed double free in spdk_rpc_add_portal_group()
spdk_iscsi_portal_create() now takes string arguments as const char* and makes internal copies of them.
This patch also fixes potential memory leak when id == NULL
Change-Id: I4d0efb101471fb2368ceb8ceecb0e40614e3585d
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>