Hpack optimizations

If the dynamic table size is already 0, do not evictAll.

Signed-off-by: Greg Wilkins <gregw@webtide.com>

jetty-http2/http2-hpack/src/main/java/org/eclipse/jetty/http2/hpack/HpackContext.java

@@ -395,6 +395,8 @@ public class HpackContext
         {
             if (LOG.isDebugEnabled())
                 LOG.debug(String.format("HdrTbl[%x] evictAll", HpackContext.this.hashCode()));
+            if (size() > 0)
+            {
                 _fieldMap.clear();
                 _nameMap.clear();
                 _offset = 0;
@@ -403,6 +405,7 @@ public class HpackContext
                 Arrays.fill(_entries, null);
             }
         }
+    }
 
     public static class Entry
     {