Oops. Fix regression in instrument loader resulting in a confusing error message.

instrument.C

@@ -51,7 +51,7 @@ using std::string;
        loading instruments, user defined instruments always hide system
        defined instruments of the same name.
 
- */
+*/
 
 list <Instrument *> Instrument::instruments;
 
@@ -290,11 +290,20 @@ Instrument::listing ( void )
     list <string> *sys = get_listing( SYSTEM_PATH INSTRUMENT_DIR );
     list <string> *usr = get_listing( config.user_config_dir );
 
+    if ( ! ( usr || sys ) )
+        return NULL;
+
+    if ( sys && usr )
         usr->merge( *sys );
+    else
+        if ( sys && ! usr )
+            usr = sys;
+
     usr->unique();
 
     usr->sort();
 
+    if ( sys )
         delete sys;
 
     char **sa = (char**)malloc( (usr->size() + 1) * sizeof( char * ) );