test: fix launch_efi()

We cannot use old_dtb in the finally block if it has not been assigned
before the try block.

test/py/tests/test_efi_fit.py:458:30:
E0601: Using variable 'old_dtb' before assignment (used-before-assignment)

Fixes: 8391f955494e ("test/py: Create a test for launching UEFI binaries from FIT images")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
1 file changed