Fix crash in editor when setting `SubViewport.size` to zero

`Viewport::_set_size` now verifies, that the minimum size is at least 2x2.
Also fix `RendererViewport::viewport_set_size` fail condition.

scene/main/viewport.cpp

@@ -823,11 +823,12 @@ void Viewport::_set_size(const Size2i &p_size, const Size2i &p_size_2d_override,
 		stretch_transform_new.scale(scale);
 	}
 
-	if (size == p_size && size_allocated == p_allocated && stretch_transform == stretch_transform_new && p_size_2d_override == size_2d_override) {
+	Size2i new_size = p_size.max(Size2i(2, 2));
+	if (size == new_size && size_allocated == p_allocated && stretch_transform == stretch_transform_new && p_size_2d_override == size_2d_override) {
 		return;
 	}
 
-	size = p_size;
+	size = new_size;
 	size_allocated = p_allocated;
 	size_2d_override = p_size_2d_override;
 	stretch_transform = stretch_transform_new;

servers/rendering/renderer_viewport.cpp

@@ -844,7 +844,7 @@ void RendererViewport::viewport_set_scaling_3d_scale(RID p_viewport, float p_sca
 }
 
 void RendererViewport::viewport_set_size(RID p_viewport, int p_width, int p_height) {
-	ERR_FAIL_COND(p_width < 0 && p_height < 0);
+	ERR_FAIL_COND(p_width < 0 || p_height < 0);
 
 	Viewport *viewport = viewport_owner.get_or_null(p_viewport);
 	ERR_FAIL_COND(!viewport);