Move throw of PackInvalidException outside the catch

When a packfile is invalid, throw an exception explicitly
outside any catch scope, so that is not accidentally caught
by the generic catch-all cause, which would set the packfile
as valid again.

Flagging an invalid packfile as valid again would have
dangerous consequences such as the corruption of the in-memory
packlist.

Bug: 544199
Change-Id: If7a3188a68d7985776b509d636d5ddf432bec798
Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackFile.java

@@ -643,9 +643,10 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 	}
 
 	private void doOpen() throws IOException {
-		try {
+		if (invalid) {
-			if (invalid)
 			throw new PackInvalidException(packFile);
+		}
+		try {
 			synchronized (readLock) {
 				fd = new RandomAccessFile(packFile, "r"); //$NON-NLS-1$
 				length = fd.length();