Fix gc_save_context when prev_stack is not pointer aligned (#475)

src/gc.c

@@ -214,6 +214,13 @@ static void gc_save_context(hl_thread_info *t, void *prev_stack ) {
 	// to gc_save_context (or before) which might hold a gc value !
 	// let's capture them immediately in extra per-thread data
 	t->stack_cur = &prev_stack;
+
+	// We have no guarantee prev_stack is pointer-aligned
+	// All calls are passing a pointer to a bool, which is aligned on 1 byte
+	// If pointer is wrongly aligned, the extra_stack_data is misaligned
+	// and register pointers save in stack will not be discovered correctly by the GC
+	uintptr_t aligned_prev_stack = ((uintptr_t)prev_stack) & ~(sizeof(void*) - 1);
+	prev_stack = (void*)aligned_prev_stack;
 	int size = (int)((char*)prev_stack - (char*)stack_cur) / sizeof(void*);
 	if( size > HL_MAX_EXTRA_STACK ) hl_fatal("GC_SAVE_CONTEXT");
 	t->extra_stack_size = size;