tools: avoid potential null pointer dereference by GCC 10

tools: avoid potential null pointer dereference by GCC 10

GCC 10 complains about "arg" possibly being a NULL dereference.
Even though it might be a false positive, we can easily avoid it.

Avoiding
 ../tools/vsh.c: In function ‘vshCommandOptStringReq’:
 ../tools/vsh.c:1034:19: error: potential null pointer dereference [-Werror=null-dereference]
  1034 |     else if (!*arg->data && !(arg->def->flags & VSH_OFLAG_EMPTY_OK))
       |                ~~~^~~~~~

Signed-off-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>