ubiupdatevol: Prevent null pointer dereference

libubi_close(libubi) is called in the error handler if libubi is null.
Prevent that by handling the error case similar to the other ubi
executables.

Signed-off-by: Bastian Germann <bastiangermann@fishpost.de>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>