Fix segfault in rs_stack_mark

I have no context over that gem, but a crash and gdb session
show us marking an empty stack:

```
(gdb) p *stack
$1 = {capacity = 0, top = 0, contents = 0x0}
```

Since we iterate with `i <= top`, we try to mark a NULL pointer
when the stack is empty.

ext/rotoscope/rotoscope.c

@@ -146,24 +146,21 @@ static void rs_dealloc(void *data) {
   xfree(config);
 }
 
-static size_t rs_memsize(const void *data) {
-  return sizeof(Rotoscope);
-}
+static size_t rs_memsize(const void *data) { return sizeof(Rotoscope); }
 
 static const rb_data_type_t rs_data_type = {
     .wrap_struct_name = "Rotoscope",
-    .function = {
-        .dmark = rs_gc_mark,
-        .dfree = rs_dealloc,
-        .dsize = rs_memsize,
-    },
-    .flags = RUBY_TYPED_FREE_IMMEDIATELY
-};
+    .function =
+        {
+            .dmark = rs_gc_mark,
+            .dfree = rs_dealloc,
+            .dsize = rs_memsize,
+        },
+    .flags = RUBY_TYPED_FREE_IMMEDIATELY};
 
 static VALUE rs_alloc(VALUE klass) {
   Rotoscope *config;
-  VALUE self =
-      TypedData_Make_Struct(klass, Rotoscope, &rs_data_type, config);
+  VALUE self = TypedData_Make_Struct(klass, Rotoscope, &rs_data_type, config);
   config->self = self;
   config->pid = getpid();
   config->tid = current_thread_id();

ext/rotoscope/stack.c

@@ -62,8 +62,10 @@ void rs_stack_init(rs_stack_t *stack, unsigned int capacity) {
 }
 
 void rs_stack_mark(rs_stack_t *stack) {
-  for (int i = 0; i <= stack->top; i++) {
-    rs_stack_frame_t *frame = &stack->contents[i];
-    rs_method_desc_mark(&frame->method);
+  if (stack->contents) {
+    for (int i = 0; i <= stack->top; i++) {
+      rs_stack_frame_t *frame = &stack->contents[i];
+      rs_method_desc_mark(&frame->method);
+    }
   }
 }