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

@@ -989,20 +989,21 @@ def sym(addr, pid, show_module=False, show_offset=False):
  returned. When show_module is True, the module name is also included.
  When show_offset is True, the instruction offset as a hexadecimal
  number is also included in the string.
- 
+
  A pid of less than zero will access the kernel symbol cache.
 
  Example output when both show_module and show_offset are True:
  "start_thread+0x202 [libpthread-2.24.so]"
- 
+
  Example output when both show_module and show_offset are False:
  "start_thread"
  """
  name, offset, module = BPF._sym_cache(pid).resolve(addr)
  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