Fix crashing when -u never parameter is given

The time_of_last_update attribute is now defined when -u never parameter is given

cve_bin_tool/cli.py

@@ -253,6 +253,7 @@ def main(argv=None):
         cvedb_orig.get_cvelist_if_stale()
     else:
         LOGGER.warning("Not verifying CVE DB cache")
+        cvedb_orig.get_db_update_date()
         if not cvedb_orig.nvd_years():
             with ErrorHandler(mode=error_mode, logger=LOGGER):
                 raise EmptyCache(cvedb_orig.cachedir)

cve_bin_tool/cvedb.py

@@ -85,10 +85,11 @@ def get_cve_count(self):
         return self.cve_count
 
     def get_db_update_date(self):
-        return os.path.getmtime(self.dbpath)
-
         # last time when CVE data was updated
-        self.time_of_last_update = datetime.datetime.now()
+        self.time_of_last_update = datetime.datetime.fromtimestamp(
+            os.path.getmtime(self.dbpath)
+        )
+        return os.path.getmtime(self.dbpath)
 
     async def getmeta(self, session, meta_url):
         async with session.get(meta_url) as response: