sipvoiplink: fix keyframe request lock

Change-Id: I514943409a319b3d7749d05e6a3ff41c4f598c24

src/sip/sipvoiplink.cpp

@@ -747,10 +747,10 @@ SIPVoIPLink::enqueueKeyframeRequest(const std::string &id)
 void
 SIPVoIPLink::dequeKeyframeRequests()
 {
+    std::lock_guard<std::mutex> lock(keyframeRequestsMutex_);
     int max_requests = 20;
 
     while (not keyframeRequests_.empty() and max_requests--) {
-        std::lock_guard<std::mutex> lock(keyframeRequestsMutex_);
         const std::string &id(keyframeRequests_.front());
         requestKeyframe(id);
         keyframeRequests_.pop();