x86_64: thread: ensure cs and ss are initialised properly in cloned thread context

arch/x86_64/thread.c

@@ -93,6 +93,8 @@ kern_status_t ml_thread_clone_user_context(
 	memcpy(regs, src_regs, sizeof *regs);
 
 	regs->rax = return_value;
+	regs->ss = 0x1b;
+	regs->cs = 0x23;
 
 	dest_ml->tr_fsbase = src_ml->tr_fsbase;
 	dest_ml->tr_gsbase = src_ml->tr_gsbase;

ds/ringbuf.c

@@ -286,8 +286,8 @@ kern_status_t ringbuf_write(
 	kern_status_t status = KERN_OK;
 
 	while (remaining > 0) {
-		void *dest;
+		void *dest = NULL;
-		size_t available;
+		size_t available = 0;
 		wait_for_capacity(buf, irq_flags);
 
 		status = ringbuf_open_write_buffer(buf, &dest, &available);