CCLabelAtlas.cpp: add check for n when call setDirty

in CCLabelAtlas::updateAtlasValues, there is the case that n equal to 0,
so we need to check it before when call m_pTextureAtlas->setDirty(true) and do
other process.

Signed-off-by: Yongqin Liu <liuyq0307@thundersoft.com>