cmd_ubi: Fix uninitialized variable warning

gcc 3.4.6 previously reported the following error on many MIPS boards
which utilize UBI:
  cmd_ubi.c:193: warning: 'vol' might be used uninitialized in this function

The current code is structured such that 'vol' will never be used when
it is NULL anyway, but gcc isn't smart enough to figure this out.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Stefan Roese <sr@denx.de>