Fix convictionPolicy connection counter go negative

DefaultConvictionPolicy.openConnections can become negative:
```
java.lang.AssertionError
at com.datastax.driver.core.ConvictionPolicy$DefaultConvictionPolicy.signalConnectionClosed(ConvictionPolicy.java:90)
at com.datastax.driver.core.Connection.closeAsync(Connection.java:953)
at com.datastax.driver.core.HostConnectionPool.discardAvailableConnections(HostConnectionPool.java:882)
at com.datastax.driver.core.HostConnectionPool.closeAsync(HostConnectionPool.java:843)
```

Reason is that signalConnectionsOpening is not guarded against negative
number in `count`

Author: dkropachev

driver-core/src/main/java/com/datastax/driver/core/Connection.java

@@ -79,6 +79,7 @@
 import java.net.ServerSocket;
 import java.security.InvalidParameterException;
 import java.text.MessageFormat;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -1330,6 +1331,9 @@ Connection open(HostConnectionPool pool, int shardId, int serverPort)
      * start them.
      */
     List<Connection> newConnections(HostConnectionPool pool, int count) {
+      if (count <= 0) {
+        return Collections.emptyList();
+      }
       pool.host.convictionPolicy.signalConnectionsOpening(count);
       List<Connection> connections = Lists.newArrayListWithCapacity(count);
       for (int i = 0; i < count; i++)