Fix 64-bit upper half power (note may still lose precision, see sjmul…

…derGH-1)

nbt.js

@@ -59,7 +59,7 @@
 			/* FIXME: this can overflow, JS has 53 bit precision */
 			var upper = this.int();
 			var lower = this.int();
-			return (upper << 32) + lower;
+			return (upper * 4294967296) + lower;
 		};
 
 		this[nbt.tagTypes.byteArray] = function() {