fixed wrong execution of bfd_get_symtab_upper_bound in pstack/pstack.c
for files which don't contain symbols..
to avoid failing of new binutils (old binutils returned 0 in this case)


pstack/pstack.c:
  fixed bug #1661 Compiling --pstack fails with binutils 2.13.90
  
  fixed wrong execution of bfd_get_symtab_upper_bound 
  for files which don't contain symbols..
  to avoid failing of new binutils (old binutils returned 0 in this case)