Dereference pointer NN, N2

[mwm126]

Fixes build error with Clang on Mac:

[ 37%] Building CXX object CMakeFiles/psuade-lib.dir/Src/FuncApprox/PsuadeRegression.cpp.o
/Users/mmeredith/repos/psuade/Src/FuncApprox/PsuadeRegression.cpp:76:10: error: ordered comparison between pointer and zero ('int *' and 'int')
  if (N2 <= 0)
      ~~ ^  ~
/Users/mmeredith/repos/psuade/Src/FuncApprox/PsuadeRegression.cpp:107:10: error: ordered comparison between pointer and zero ('int *' and 'int')
  if (NN <= 0)
      ~~ ^  ~
/Users/mmeredith/repos/psuade/Src/FuncApprox/PsuadeRegression.cpp:145:10: error: ordered comparison between pointer and zero ('int *' and 'int')
  if (NN <= 0)
      ~~ ^  ~
/Users/mmeredith/repos/psuade/Src/FuncApprox/PsuadeRegression.cpp:192:10: error: ordered comparison between pointer and zero ('int *' and 'int')
  if (NN <= 0)
      ~~ ^  ~
/Users/mmeredith/repos/psuade/Src/FuncApprox/PsuadeRegression.cpp:245:10: error: ordered comparison between pointer and zero ('int *' and 'int')
  if (NN <= 0)
      ~~ ^  ~
5 errors generated.