normal/completion: Fix leaking of memory when processing a completion

normal/completion: Fix leaking of memory when processing a completion

It is possible for the code to reach the end of the function without
freeing the memory allocated to argv and argc still to be 0.

We should always call grub_free(argv). The grub_free() will handle
a NULL argument correctly if it reaches that code without the memory
being allocated.

Fixes: CID 96672
Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>