vquic: make vquic_send_packets not return without setting psent

This is debug code, but since the caller might use the value even when
this function returns error, it needs to be cleared properly here.

Spotted by Clang-tidy

Closes #15807

lib/vquic/vquic.c

@@ -248,6 +248,7 @@ static CURLcode vquic_send_packets(struct Curl_cfilter *cf,
   /* simulate network blocking/partial writes */
   if(qctx->wblock_percent > 0) {
     unsigned char c;
+    *psent = 0;
     Curl_rand(data, &c, 1);
     if(c >= ((100-qctx->wblock_percent)*256/100)) {
       CURL_TRC_CF(data, cf, "vquic_flush() simulate EWOULDBLOCK");