Error with clang++ in building bpls

Created by: jychoi-hpc

https://github.com/ornladios/ADIOS2/blob/c9dfa1d326995e2c5eb8fae53b53a3e6c3446efe/source/utils/bpls/bpls.cpp#L527

Just as a quick report, with clang++, I got the following error:

error: ordered comparison between pointer and zero
      ('adios2::Engine *' and 'int')
    if (fp > 0)
        ~~ ^ ~

Maybe we need to use (fp != NULL)?