NPE during RedissonPermitExpirableSemaphore.availablePermits method call fixed

redisson/src/main/java/org/redisson/RedissonPermitExpirableSemaphore.java

@@ -672,7 +672,8 @@ public class RedissonPermitExpirableSemaphore extends RedissonExpirable implemen
                     "end;" + 
                     "return value; " +
                 "end; " +
-                "return redis.call('get', KEYS[1]); ",
+                "local ret = redis.call('get', KEYS[1]); " + 
+                "return ret == false and 0 or ret;",
                 Arrays.<Object>asList(getName(), timeoutName, getChannelName()), System.currentTimeMillis());
     }
 

redisson/src/test/java/org/redisson/RedissonPermitExpirableSemaphoreTest.java

@@ -13,6 +13,12 @@ import org.redisson.client.RedisException;
 
 public class RedissonPermitExpirableSemaphoreTest extends BaseConcurrentTest {
 
+    @Test
+    public void testNotExistent() {
+        RPermitExpirableSemaphore semaphore = redisson.getPermitExpirableSemaphore("testSemaphoreForNPE");
+        Assert.assertEquals(0, semaphore.availablePermits());        
+    }
+    
     @Test
     public void testAvailablePermits() throws InterruptedException {
         RPermitExpirableSemaphore semaphore = redisson.getPermitExpirableSemaphore("test-semaphore");