test: fit: Using variable 'old_dtb' before assignment

old_dtb can only be assumed initialized in the finally block
if it is assigned a value before the try statement.

Avoid a pylint error reported by current pylint.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed