Fixed crashes and instability when love.graphics.setMode is called when a vertex buffer is mapped by love.

src/modules/graphics/opengl/VertexBuffer.cpp

@@ -291,13 +291,11 @@ void VBO::unload(bool save)
 	if (save && getMemoryBacking() == BACKING_PARTIAL)
 	{
 		VertexBuffer::Bind bind(*this);
-
-		bool mapped = is_mapped;
-
 		map(); // saves buffer content to memory_map.
-		is_mapped = mapped;
 	}
 
+	is_mapped = false;
+
 	glDeleteBuffersARB(1, &vbo);
 	vbo = 0;
 }