Fix crashing when trying to replace snow. (Pulled from 1.12)

src/main/java/com/progwml6/natura/world/worldgen/trees/overworld/OverworldTreeGenerator.java

@@ -229,7 +229,7 @@ protected void placeTrunk(World world, BlockPos pos, int height)
             IBlockState state = world.getBlockState(blockpos);
             Block block = state.getBlock();
 
-            if (block.isAir(state, world, blockpos) || block == null || block.isLeaves(state, world, blockpos))
+            if (block == null || block.isAir(state, world, blockpos) || block.isLeaves(state, world, blockpos) || block.isReplaceable(world, blockpos))
             {
                 world.setBlockState(blockpos, this.log, 2);
             }