Add some sanity checks to Palette.readFromNBT

Printers use a Terminal to store the page currently being printed.
Printers saved in an older version of ComputerCraft would be missing the
term_palette field, resulting in an NPE when loading the tile.

src/main/java/dan200/computercraft/shared/util/Palette.java

@@ -89,8 +89,11 @@ public class Palette
 
     public void readFromNBT( NBTTagCompound nbt )
     {
+        if( !nbt.hasKey( "term_palette" ) ) return;
         int[] rgb8 = nbt.getIntArray( "term_palette" );
 
+        if( rgb8.length != colours.length ) return;
+
         for(int i = 0; i < colours.length; ++i)
         {
             colours[i] = decodeRGB8( rgb8[i] );