avoid segmentation fault, when creating a LAMMPS instance from the library interface. arg[] may be NULL.

src/lammps.cpp

@@ -89,7 +89,8 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator)
 
   suffix = suffix2 = NULL;
   suffix_enable = 0;
-  exename = arg[0];
+  if (arg) exename = arg[0];
+  else exename = (const char *)"(LAMMPS)";
   packargs = NULL;
   num_package = 0;
   char *rfile = NULL;