Update TreePacksPanel.java

Size computation in the TreePacksPanel can give negative numbers due to a cast of a long into an int.
This has been reproduced with an installer with 5 nodes (139.43 Mb, 24.54 Mb, 2.81 Mb, 3.65Mb and 1.88 Gb): selecting all the nodes give the result of -2095174524 bytes for the "Total space required" instead of 2.05 Gb.
Merge pull request #107 from ctranxuan/master

Invalid size computation in the TreePacksPanel (negative number)