vty: Use NULL to have a null pointer instead of '\0'

'\0' gets translated to zero but the argument to vector_set is
a pointer and it gets converted to a pointer.

vty.c:985:21: warning: expression which evaluates to zero treated as a null pointer constant of type
      'void *' [-Wnon-literal-null-conversion]
                vector_set(vline, '\0');
                                  ^~~~
vty.c:1095:21: warning: expression which evaluates to zero treated as a null pointer constant of type
      'void *' [-Wnon-literal-null-conversion]
                vector_set(vline, '\0');
                                  ^~~~
vty.c:1097:21: warning: expression which evaluates to zero treated as a null pointer constant of type
      'void *' [-Wnon-literal-null-conversion]
                vector_set(vline, '\0');
                                  ^~~~

0 files changed, 0 insertions, 0 deletions