tools: Fix pylint 3.3.4 errors

This newer pylint produces errors about variables possibly being used
before being set. Adjust the code to pass these checks.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Tom Rini <trini@konsulko.com>

tools/binman/ftest.py

@@ -6381,6 +6381,7 @@ fdt         fdtmap                Extract the devicetree blob from the fdtmap
                     ename, prop = entry_m.group(1), entry_m.group(3)
                 entry, entry_name, prop_name = image.LookupEntry(entries,
                                                                  name, msg)
+                expect_val = None
                 if prop_name == 'offset':
                     expect_val = entry.offset
                 elif prop_name == 'image_pos':