Prevent :LinediffReset command from failing if plugin is uninitialized

Don't try to reset anything if nothing is initialized.

The issue can be reproduced by running `:LinediffReset` command before `:Linediff` in Vim's session, in which case I got:
```
Error detected while processing function <SNR>41_LinediffReset:
line    1:
E121: Undefined variable: s:differ_one
line    2:
E121: Undefined variable: s:differ_two
```