DfsBlockCache#creditSpace: release clockLock in finally block

Enclose the call to getStat in a `try`, and release the previously
acquired lock in the `finally`. This prevents that the lock is left
unreleased in the case of an exception being raised in getStat.

Change-Id: I17b4cd134dae887e23a1165253be0ac2d4fd452c
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCache.java

@@ -418,9 +418,12 @@ public final class DfsBlockCache {
 
 	private void creditSpace(int credit, DfsStreamKey key) {
 		clockLock.lock();
+		try {
 			getStat(liveBytes, key).addAndGet(-credit);
+		} finally {
 			clockLock.unlock();
 		}
+	}
 
 	@SuppressWarnings("unchecked")
 	private void addToClock(Ref ref, int credit) {