[process] Hold reference to process during call to step()

It is conceivable that the process may terminate during the execution
of step().  If nothing else holds a reference to the containing
object, this would cause the object to be freed prior to returning
from step().

Add a ref_get()/ref_put() around the call to ->step() to prevent this
from happening.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/process.c

 	list_for_each_entry ( process, &run_queue, list ) {
 		list_del ( &process->list );
 		list_add_tail ( &process->list, &run_queue );
+		ref_get ( process->refcnt ); /* Inhibit destruction mid-step */
 		DBGC2 ( process, "PROCESS %p executing\n", process );
 		process->step ( process );
 		DBGC2 ( process, "PROCESS %p finished executing\n", process );
+		ref_put ( process->refcnt ); /* Allow destruction */
 		break;
 	}
 }