Mixer: Fix an invalid read on strip destruction due to a missing lock.

mixer/src/Mixer_Strip.C

@@ -94,6 +94,7 @@ Mixer_Strip::~Mixer_Strip ( )
 {
     DMESSAGE( "Destroying mixer strip" );
 
+    _chain->engine()->lock();
 
     /* make sure this gets destroyed before the chain */
     fader_tab->clear();

mixer/src/Module.C

@@ -71,6 +71,8 @@ Module::Module ( ) : Fl_Group( 0, 0, 50, 50, "Unnamed" )
 
 Module::~Module ( )
 {
+    /* we assume that the engine for this chain is already locked */
+
     if ( _editor )
     {
         delete _editor;