Fixes #591 Synchronise the track statement method, and clearing of the tracked statement list.

src/main/java/com/zaxxer/hikari/pool/ProxyConnection.java

@@ -182,7 +182,7 @@ public abstract class ProxyConnection implements Connection
       leakTask.cancel();
    }
 
-   private final <T extends Statement> T trackStatement(final T statement)
+   private final synchronized <T extends Statement> T trackStatement(final T statement)
    {
       openStatements.add(statement);
 
@@ -205,9 +205,11 @@ public abstract class ProxyConnection implements Connection
             }
          }
 
+         synchronized (this) {
             openStatements.clear();
          }
       }
+   }
 
    // **********************************************************************
    //              "Overridden" java.sql.Connection Methods

src/main/java/com/zaxxer/hikari/util/FastList.java

@@ -184,6 +184,10 @@ public final class FastList<T> extends ArrayList<T>
    @Override
    public T remove(int index)
    {
+      if (size == 0) {
+         return null;
+      }
+
       final T old = elementData[index];
 
       final int numMoved = size - index - 1;