[kernel] Fix current thread deletion bug

Defer from calling process::thread_exited() in scheduler::prune() if the
thread in question is the currently-executing thread, so that we don't
blow away the stack we're executing on. The next call to prune will pick
up the exited thread.

src/kernel/frame_allocator.cpp

@@ -45,6 +45,8 @@ inline uintptr_t end(frame_block *node) { return node->address + node->count * f
 void
 frame_allocator::free(uintptr_t address, size_t count)
 {
+	kassert(address % frame_size == 0, "Trying to free a non page-aligned frame!");
+
 	frame_block_node *node =
 		reinterpret_cast<frame_block_node*>(address + page_offset);
 

src/kernel/scheduler.cpp

@@ -241,6 +241,7 @@ void scheduler::prune(uint64_t now)
 		bool ready = th->has_state(thread::state::ready);
 		bool exited = th->has_state(thread::state::exited);
 		bool constant = th->has_state(thread::state::constant);
+		bool current = tcb == m_current;
 
 		ready |= th->wake_on_time(now);
 
@@ -253,7 +254,10 @@ void scheduler::prune(uint64_t now)
 
 		if (exited) {
 			process &p = th->parent();
-			if(p.thread_exited(th))
+
+			// If the current thread has exited, wait until the next call
+			// to prune() to delete it.
+			if(!current && p.thread_exited(th))
 				delete &p;
 		} else {
 			log::debug(logs::task, "Prune: readying unblocked thread %llx", th->koid());