run: don't pass extra null argument

Fixes #47
Check size of the string before calling split on it, else
we end up adding a null argument to the command.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>