http2: move state-init from creation to pre-transfer

To make sure that the HTTP/2 state is initialized correctly for
duplicated handles. It would otherwise easily generate "spurious"
PRIORITY frames to get sent over HTTP/2 connections when duplicated easy
handles were used.

Reported-by: Daniel Silverstone
Fixes #4303
Closes #4442

lib/transfer.c

@@ -1510,6 +1510,7 @@ CURLcode Curl_pretransfer(struct Curl_easy *data)
       }
     }
 #endif
+    Curl_http2_init_state(&data->state);
   }
 
   return result;