Clear another FindBugs warning in SPVBlockStore.

The analysis doesn't seem to be good enough to detect that some of these cases are safe.

core/src/main/java/com/google/bitcoin/store/SPVBlockStore.java

@@ -145,8 +145,11 @@ public class SPVBlockStore implements BlockStore {
         buffer.put(header);
         // Insert the genesis block.
         lock.lock();
+        try {
             setRingCursor(buffer, FILE_PROLOGUE_BYTES);
+        } finally {
             lock.unlock();
+        }
         Block genesis = params.genesisBlock.cloneAsHeader();
         StoredBlock storedGenesis = new StoredBlock(genesis, genesis.getWork(), 0);
         put(storedGenesis);