Update mesh.h

Fix a typo.

include/assimp/mesh.h

@@ -989,7 +989,7 @@ struct aiMesh {
             // Construct and null-init array
             mTextureCoordsNames = new aiString *[AI_MAX_NUMBER_OF_TEXTURECOORDS];
             for (size_t i=0; i<AI_MAX_NUMBER_OF_TEXTURECOORDS; ++i) {
-                mTextureCoordsNames = nullptr;
+                mTextureCoordsNames[i] = nullptr;
             }
         }