ts_31_102: Avoid pylint false positive

This should avoid the following pylint error:

************* Module pySim.ts_31_102
pySim/ts_31_102.py:621:100: E0601: Using variable 'sw' before assignment (used-before-assignment)

Change-Id: I0bb9607cdab0e6e3cd17b4d27129a51a607bc0f2

pySim/ts_31_102.py

@@ -609,6 +609,7 @@ class EF_UServiceTable(TransparentEF):
                     should_exist = f.should_exist_for_services(active_services)
                     try:
                         cmd.rs.select_file(f)
+                        sw = None
                         exists = True
                     except SwMatchError as e:
                         sw = str(e)