Don't mark FastVector element as used until it's ready.

This should prevent a race condition that would result in invalid last
element of the queue, in case a freezed thread already got the queue
item, but didn't wrote to it (or didn't wrote fully).

client/TracyFastVector.hpp

@@ -60,6 +60,17 @@ public:
         return m_write++;
     }
 
+    T* prepare_next()
+    {
+        if( m_write == m_end ) AllocMore();
+        return m_write;
+    }
+
+    void commit_next()
+    {
+        m_write++;
+    }
+
     void clear()
     {
         m_write = m_ptr;