Fix that corrects a mishandling of UInt64's.

mcs/class/corlib/Mono/DataConverter.cs

@@ -1194,7 +1194,7 @@ public override void PutBytes (byte [] dest, int destIdx, ulong value)
 				fixed (byte *target = &dest [destIdx]){
 					byte *source = (byte *) &value;
 
-					for (int i = 0; i < 4; i++)
+					for (int i = 0; i < 8; i++)
 						target [i] = source [7-i];
 				}
 			}