mrtgInit() is not checking if filename parameter is NULL so
when dvblast is run without -Z option that sets mrtg file
the following error is seen in valgrind:

 Syscall param open(filename) points to unaddressable byte(s)
    at 0x412798E: __open_nocancel (in /lib/libc-2.13.so)
    by 0x40BCA33: __fopen_internal (in /lib/libc-2.13.so)
    by 0x40BCA9B: fopen@@GLIBC_2.1 (in /lib/libc-2.13.so)
    by 0x8062841: mrtgInit (mrtg-cnt.c:167)
    by 0x4073DB5: (below main) (in /lib/libc-2.13.so)