OGL/VertexManager: Minor cleanup of if case
OGL/VertexManager: Do not check if g_nativeVertexFmt is zero

The variable is already dereferenced both before and after this
check which means that if this variable would ever be zero it would
have crashed dolphin already.
Merge pull request #191 from Jezze/for-dolphin

Some cleanups for OGL/Vertexmanager.cpp