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

(cherry picked from commit 1731805967)

rust/src/http2/http2.rs

@@ -492,6 +492,7 @@ impl HTTP2Transaction {
                         SCAppLayerForceProtocolChange(flow, ALPROTO_DOH2);
                     }
                 }
+                doh.data_buf[dir.index()].clear();
             }
         }
     }