inspector: add missing initialization

Add missing initialization flagged by Coverity

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: nodejs#41022
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/cctest/test_inspector_socket_server.cc

@@ -354,6 +354,7 @@ class TestSocketServerDelegate : public SocketServerDelegate {
 
 ServerHolder::ServerHolder(bool has_targets, uv_loop_t* loop,
                            const std::string& host, int port, FILE* out) {
+  session_id_ = 0;
   std::vector<std::string> targets;
   if (has_targets)
     targets = { MAIN_TARGET_ID };