Reject nan as table index.

src/core/org/luaj/vm2/LuaDouble.java

@@ -192,4 +192,9 @@ public class LuaDouble extends LuaNumber {
 		return LuaString.valueOf(tojstring());
 	}
 	
+	public LuaValue checkvalidkey() {
+		if ( Double.isNaN(v) )
+			throw new LuaError("table index expected, got nan");
+		return this; 
+	}	
 }

src/core/org/luaj/vm2/LuaNil.java

@@ -63,6 +63,10 @@ public class LuaNil extends LuaValue {
 		return typerror("value");
 	}
 	
+	public LuaValue checkvalidkey() {
+		return typerror("table index"); 
+	}
+
 	// optional argument conversions - nil alwas falls badk to default value
 	public boolean     optboolean(boolean defval)          { return defval; }
 	public LuaClosure  optclosure(LuaClosure defval)       { return defval; }

src/core/org/luaj/vm2/LuaTable.java

@@ -189,7 +189,7 @@ public class LuaTable extends LuaValue {
 
 	/** caller must ensure key is not nil */
 	public void set( LuaValue key, LuaValue value ) {
-		key.checknotnil();
+		key.checkvalidkey();
 		if ( m_metatable==null || ! rawget(key).isnil() ||  ! settable(this,key,value) )
 			rawset(key, value);
 	}

src/core/org/luaj/vm2/LuaValue.java

@@ -153,6 +153,8 @@ public class LuaValue extends Varargs {
 	public Object      checkuserdata()         { typerror("userdata");  return null; }
 	public Object      checkuserdata(Class c)  { typerror("userdata");  return null; }
 	public LuaValue    checknotnil()           { return this; }
+	public LuaValue    checkvalidkey()         { return this; }
+
 	
 	/** @deprecated - use checkjstring() instead */
 	public String      checkString()          { return checkjstring(); }