[http3] Revert connection count on Initial decryption failure
[http3] Revert connection count on Initial decryption failure

Upon receiving a QUIC Initial packet, the current logic first increments
the connection counter, but fails to decrement it when it turns out to be
a non-decryptable Initial. This would cause a "leak" of the counter when
such a packet is received.

cwyang/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,cwyang/h2o,h2o/h2o,cwyang/h2o,devnexen/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,h2o/h2o,yannick/h2o,h2o/h2o,h2o/h2o,devnexen/h2o