trivial: debug code for SH-2827: [crashhunters] Crash in LLPipeline::generateWaterReflection()

indra/llrender/llrender.cpp

@@ -1434,6 +1434,8 @@ void LLRender::loadIdentity()
 	flush();
 
 	{
+		llassert_always(mMatrixMode < NUM_MATRIX_MODES) ;
+
 		mMatrix[mMatrixMode][mMatIdx[mMatrixMode]].make_identity();
 		mMatHash[mMatrixMode]++;
 	}