more fixes for coverity-findings.

test/unit/utPLYImportExport.cpp

@@ -56,6 +56,9 @@ public:
         const aiScene *scene = importer.ReadFile( ASSIMP_TEST_MODELS_DIR "/PLY/cube.ply", aiProcess_ValidateDataStructure);
         EXPECT_EQ( 1u, scene->mNumMeshes );
         EXPECT_NE( nullptr, scene->mMeshes[0] );
+        if (nullptr == scene->mMeshes[0]) {
+            return false;
+        }
         EXPECT_EQ( 8u, scene->mMeshes[0]->mNumVertices );
         EXPECT_EQ( 6u, scene->mMeshes[0]->mNumFaces );