Permit null target on TCL connection

In previous versions of OpenOCD, it was possible to connect to the TCL
RPC interface without a current target. In `tcl_new_connection`, the
curent target would be queried by number, and the possibility of a null
current target was handled properly.

In commit bb9d9c6026, the
`get_target_by_num` call was replaced by a `get_current_target` call,
without noticing that `get_current_target` aborts if there is no current
target, whereas `tcl_new_connection` is perfectly able to handle that
situation.

Provide a `get_current_target_or_null` function for use by consumers who
are OK with a null current target, and use it in `tcl_new_connection`.

Change-Id: I06f7e1e149f1169e23c73ba328c7ad9f9425cc2a
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4730
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Franck Jullien <franck.jullien@gmail.com>

src/server/tcl_server.c

@@ -157,7 +157,7 @@ static int tcl_new_connection(struct connection *connection)
 
 	connection->priv = tclc;
 
-	struct target *target = get_current_target(connection->cmd_ctx);
+	struct target *target = get_current_target_or_null(connection->cmd_ctx);
 	if (target != NULL)
 		tclc->tc_laststate = target->state;
 

src/target/target.c

@@ -517,9 +517,7 @@ struct target *get_target_by_num(int num)
 
 struct target *get_current_target(struct command_context *cmd_ctx)
 {
-	struct target *target = cmd_ctx->current_target_override
+	struct target *target = get_current_target_or_null(cmd_ctx);
-		? cmd_ctx->current_target_override
-		: cmd_ctx->current_target;
 
 	if (target == NULL) {
 		LOG_ERROR("BUG: current_target out of bounds");
@@ -529,6 +527,13 @@ struct target *get_current_target(struct command_context *cmd_ctx)
 	return target;
 }
 
+struct target *get_current_target_or_null(struct command_context *cmd_ctx)
+{
+	return cmd_ctx->current_target_override
+		? cmd_ctx->current_target_override
+		: cmd_ctx->current_target;
+}
+
 int target_poll(struct target *target)
 {
 	int retval;

src/target/target.h

@@ -396,6 +396,7 @@ int target_call_timer_callbacks_now(void);
 
 struct target *get_target_by_num(int num);
 struct target *get_current_target(struct command_context *cmd_ctx);
+struct target *get_current_target_or_null(struct command_context *cmd_ctx);
 struct target *get_target(const char *id);
 
 /**