doh2: clear the buffer after processing it

Ticket: 8725

So that multiple HTTP2 DATA frames with EndOfStream flag set,
do not make the buffer grow, while processing it each time,
resulting in quadratic complexity

rust/src/http2/http2.rs

@@ -484,6 +484,7 @@ impl HTTP2Transaction {
                         SCAppLayerForceProtocolChange(flow, ALPROTO_DOH2);
                     }
                 }
+                doh.data_buf[dir.index()].clear();
             }
         }
     }