message: On frame save, unmap only if the actual pointer is non-null.
diff: @@ -257,11 +257,11 @@ void FrameBuffer::saveFrame(std::string filename, int flags)
  exp->doExport();
  unmap(fb);
  - if (albedo)
  + if (abuf)
  unmap(abuf);
  - if (depth)
  + if (zbuf)
  unmap(zbuf);
  - if (normal)
  + if (nbuf)
  unmap(nbuf);
  }
  