target/pr: Fix possible uninitialized variable usage

Triggered a compilation warning.

Fixes: 2650d71e target: move transport ID handling to the core
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_pr.c

@@ -1474,7 +1474,7 @@ core_scsi3_decode_spec_i_port(
 	LIST_HEAD(tid_dest_list);
 	struct pr_transport_id_holder *tidh_new, *tidh, *tidh_tmp;
 	unsigned char *buf, *ptr, proto_ident;
-	const unsigned char *i_str;
+	const unsigned char *i_str = NULL;
 	char *iport_ptr = NULL, i_buf[PR_REG_ISID_ID_LEN];
 	sense_reason_t ret;
 	u32 tpdl, tid_len = 0;