python: handle null module in BPF.sym

Check to see if `module` is None before attempting to call `os.path.basename`
on it. Before:

```
>>> BPF.sym(0x400001, 12345, show_module=True)
Traceback (most recent call last):
..
AttributeError: 'NoneType' object has no attribute 'rfind'
```

After:

```
>>> BPF.sym(0x400001, 12345, show_module=True)
'[unknown]'
```

src/python/bcc/__init__.py

@@ -1002,7 +1002,8 @@ class BPF(object):
         offset = "+0x%x" % offset if show_offset and name is not None else ""
         name = name or "[unknown]"
         name = name + offset
-        module = " [%s]" % os.path.basename(module) if show_module else ""
+        module = " [%s]" % os.path.basename(module) \
+            if show_module and module is not None else ""
         return name + module
 
     @staticmethod