fixed comparison error

[noguchi-k]

Hello.I found an error building denovogear, so I suggest sourse fixes.
Could you check if it is the right fix?

• error

  >> 162    newmat6.cpp:431:13: error: ordered comparison between pointer and z
            ero ('int *' and 'int')                                            
     163       if (indx > 0) { delete [] indx; indx = 0; }
     164           ~~~~ ^ ~

I changed comparison from indx > 0 to indx != NULL