don't initialize twice

pandatool/src/mayaprogs/mayaCopy.cxx

@@ -149,10 +149,12 @@ filter_filename(const string &source) {
 bool MayaCopy::
 copy_maya_file(const Filename &source, const Filename &dest,
                CVSSourceDirectory *dir) {
-  _maya = MayaApi::open_api(_program_name);
-  if (!_maya->is_valid()) {
-    nout << "Unable to initialize Maya.\n";
-    exit(1);
+  if (_maya.is_null()) {
+    _maya = MayaApi::open_api(_program_name);
+    if (!_maya->is_valid()) {
+      nout << "Unable to initialize Maya.\n";
+      exit(1);
+    }
   }
 
   if (!_maya->read(source)) {