fix ArithmeticException "/ by zero"

Signed-off-by: Ludovic Orban <lorban@bitronix.be>

jetty-util/src/main/java/org/eclipse/jetty/util/Pool.java

@@ -249,6 +249,11 @@ public class Pool<T> implements AutoCloseable, Dumpable
             {
                 LOGGER.trace("IGNORED", e);
                 size = entries.size();
+                // Size can be 0 when the pool is in the middle of
+                // acquiring a connection while another thread
+                // removes the last one from the pool.
+                if (size == 0)
+                    break;
             }
             index = (index + 1) % size;
         }