video sink: fix data race

possible data race fixed in dtor of ShmHolder.

Coverity CID # 1296815

Change-Id: I85c8158ec8fd8ada8feb28720350f2237a1097f3
Tuleap: #909

src/media/video/sinkclient.cpp

@@ -157,8 +157,11 @@ ShmHolder::~ShmHolder()
     if (area_ == MAP_FAILED)
         return;
 
+    ::sem_wait(&area_->mutex);
     area_->frameSize = 0;
-    ::sem_post(&area_->frameGenMutex);
+    ::sem_post(&area_->mutex);
+
+    ::sem_post(&area_->frameGenMutex); // unlock waiting client before leaving
     unMapShmArea();
 }