hashlib_new_insecure_functions plugin fails to parse dynamic code

[pshchelo]

Describe the bug
Running this plugin against a hashlib.new invocation that does accept the hashing algo dynamically from an inline function of a variable fails with
AttributeError: 'NoneType' object has no attribute 'lower'

To Reproduce
minimal repro module test_hash_new.py

import hashlib

def bandit_fail(algo):
    return hashlib.new(str(algo))

Run bandit >=1.5.0 against it (that's where that check plugin appeared first time)

$ bandit test_hash_new.py
...
[tester]        ERROR   Bandit internal error running: hashlib_new on file test_hash_new.py at line 4: 'NoneType' object has no attribute 'lower'Traceback (most recent call last):                                                                                                       
  File "/home/pshchelo/.virtualenvs/bandit/lib/python3.6/site-packages/bandit/core/tester.py", line 64, in run_tests                                                                                                                                                                      
    result = test(context)                                                                                                                                                                                                                                                                
  File "/home/pshchelo/.virtualenvs/bandit/lib/python3.6/site-packages/bandit/plugins/hashlib_new_insecure_functions.py", line 57, in hashlib_new                                                                                                                                         
    if name.lower() in ('md4', 'md5'):
AttributeError: 'NoneType' object has no attribute 'lower'
...

Note that moving str out of call to hashlib.new solves the issue.

Expected behavior
At least no failure/traceback. Possibly the same error emitted but with lower confidence stating something like "Verify that no insecure algos are being passed here".

Bandit version

bandit 1.6.0
  python version = 3.6.5 (default, Apr  1 2018, 05:46:30) [GCC 7.3.0]

but observed on any bandit 1.5.x too, where this check plugin is already available

Additional context
Seems like aftermath of implementing #122

Code where this very failure was observed is OpenStack's glance_store drivers, for example
https://opendev.org/openstack/glance_store/src/tag/0.29.0/glance_store/_drivers/cinder.py#L676

It seems something like

            if name and name.lower() in ('md4', 'md5'):

would suffice to fix it.

[ehooo]

This is the PR #516