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: 8391f95549 ("test/py: Create a test for launching UEFI binaries from FIT images")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

test/py/tests/test_efi_fit.py

@@ -433,11 +433,13 @@ def test_efi_fit_launch(u_boot_console):
     sys_arch = cons.config.buildconfig.get('config_sys_arch', '"sandbox"')[1:-1]
     is_sandbox = sys_arch == 'sandbox'
 
+    if is_sandbox:
+        old_dtb = cons.config.dtb
+
     try:
         if is_sandbox:
             # Use our own device tree file, will be restored afterwards.
             control_dtb = make_dtb('internal', False)
-            old_dtb = cons.config.dtb
             cons.config.dtb = control_dtb
 
         # Run tests