add null check for mSurface at ReplaseSurface

add null check for mSurface at ReplaseSurface

crash occurs :
(gdb) bt full
#0  0xb7fbb868 in ?? ()
No symbol table info available.
#1  0xb6ded69c in Dali::Internal::Adaptor::RenderHelper::ReplaceSurface (this=this@entry=0xb7fd2384, newSurface=0xb7fcd990) at ../../../dali/internal/graphics/common/render-helper.cpp:112
No locals.
#2  0xb6de877c in Dali::Internal::Adaptor::CombinedUpdateRenderController::UpdateRenderThread (this=0xb7fd2368) at ../../../dali/internal/adaptor/common/combined-update-render-controller.cpp:439
(gdb) f 1
#1  0xb6ded69c in Dali::Internal::Adaptor::RenderHelper::ReplaceSurface (this=this@entry=0xb7fd2384, newSurface=0xb7fcd990) at ../../../dali/internal/graphics/common/render-helper.cpp:112
112	../../../dali/internal/graphics/common/render-helper.cpp: No such file or directory.
(gdb) p mSurface
$1 = (Dali::RenderSurface *) 0x0

Change-Id: I23a11c968d7133e95208cbd7e9341541e3466541