Fix unbalanced new/free() -> new/delete

(Spotted while valgrinding an unrelated issue)

Change-Id: I2eab4c08e251d79f427fd01442c4dce20d7d89f0
Reviewed-on: https://gerrit.libreoffice.org/11785Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>

sal/osl/unx/thread.cxx

@@ -166,7 +166,7 @@ static void osl_thread_destruct_Impl (Thread_Impl ** ppImpl)
         pthread_cond_destroy  (&((*ppImpl)->m_Cond));
         pthread_mutex_destroy (&((*ppImpl)->m_Lock));
 
-        free (*ppImpl);
+        delete *ppImpl;
         (*ppImpl) = 0;
     }
 }