rtp: fix RtpSession::restartSender()

Calling this API causes a race condition with RtpSession::start().
The former needs the latter to be run first, this was not enforced,
causing a crash by nullptr access on socketPair_ member.
This patch checks for a non-nullptr socketPair_ to procceed the restart.

Issue: #81987
Change-Id: I20886fe70398f018621aade1c7bac3ff15ef9055

src/media/audio/audio_rtp_session.cpp

@@ -392,6 +392,10 @@ void
 AudioRtpSession::restartSender()
 {
     std::lock_guard<std::recursive_mutex> lock(mutex_);
+    // ensure that start has been called before restart
+    if (not socketPair_)
+        return;
+
     startSender();
 }
 

src/media/video/video_rtp_session.cpp

@@ -114,6 +114,10 @@ VideoRtpSession::restartSender()
 {
     std::lock_guard<std::recursive_mutex> lock(mutex_);
 
+    // ensure that start has been called before restart
+    if (not socketPair_)
+        return;
+
     startSender();
     setupVideoPipeline();
 }