src: Fix error reported by coverity scan

src/h2load_http2_session.cc

@@ -83,6 +83,9 @@ int on_stream_close_callback(nghttp2_session *session, int32_t stream_id,
   auto client = static_cast<Client *>(user_data);
   auto req_stat = static_cast<RequestStat *>(
       nghttp2_session_get_stream_user_data(session, stream_id));
+  if (!req_stat) {
+    return 0;
+  }
   client->on_stream_close(stream_id, error_code == NGHTTP2_NO_ERROR, req_stat);
   return 0;
 }
@@ -100,6 +103,7 @@ int before_frame_send_callback(nghttp2_session *session,
   client->on_request(frame->hd.stream_id);
   auto req_stat = static_cast<RequestStat *>(
       nghttp2_session_get_stream_user_data(session, frame->hd.stream_id));
+  assert(req_stat);
   client->record_request_time(req_stat);
 
   return 0;

src/nghttp.cc

@@ -1825,6 +1825,7 @@ int before_frame_send_callback(nghttp2_session *session,
   }
   auto req = static_cast<Request *>(
       nghttp2_session_get_stream_user_data(session, frame->hd.stream_id));
+  assert(req);
   req->record_request_time();
   return 0;
 }

src/shrpx_connection.cc

@@ -253,12 +253,13 @@ ssize_t Connection::read_tls(void *data, size_t len) {
 }
 
 ssize_t Connection::write_clear(const void *data, size_t len) {
-  ssize_t nwrite = std::min(len, wlimit.avail());
-  if (nwrite == 0) {
+  len = std::min(len, wlimit.avail());
+  if (len == 0) {
     return 0;
   }
 
-  while ((nwrite = write(fd, data, nwrite)) == -1 && errno == EINTR)
+  ssize_t nwrite;
+  while ((nwrite = write(fd, data, len)) == -1 && errno == EINTR)
     ;
   if (nwrite == -1) {
     if (errno == EAGAIN || errno == EWOULDBLOCK) {
@@ -298,12 +299,13 @@ ssize_t Connection::writev_clear(struct iovec *iov, int iovcnt) {
 }
 
 ssize_t Connection::read_clear(void *data, size_t len) {
-  ssize_t nread = std::min(len, rlimit.avail());
-  if (nread == 0) {
+  len = std::min(len, rlimit.avail());
+  if (len == 0) {
     return 0;
   }
 
-  while ((nread = read(fd, data, nread)) == -1 && errno == EINTR)
+  ssize_t nread;
+  while ((nread = read(fd, data, len)) == -1 && errno == EINTR)
     ;
   if (nread == -1) {
     if (errno == EAGAIN || errno == EWOULDBLOCK) {