dtoolutil: Fix typo in pfstream causing compiler warning

dtool/src/dtoolutil/pfstreamBuf.cxx

@@ -183,7 +183,7 @@ is_open() const {
  */
 bool PipeStreamBuf::
 eof_pipe() const {
-  return (_pipe == nullptr) && feof(_pipe);
+  return (_pipe == nullptr) || feof(_pipe);
 }
 
 /**