Fix sniffer join waiting indefinitely

[louis-6wind]

When trying to stop a sniffer that was not already started, Scapy_Exception("Unsupported (offline or unsupported socket)") is no more raised and join() is waiting indefinitely.

Restore the previous stop_cb behavior.

Fixes: 6689da4 ("Add missing type in AsyncSniffer (#4665)")

Checklist:

• If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• I squashed commits belonging together
• I added unit tests or explained why they are not relevant
• [] I executed the regression tests (using cd test && ./run_tests or tox)
• If the PR is still not finished, please create a Draft Pull Request

fixes #xxx

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.02%. Comparing base (bd7f2a3) to head (fd95d44).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4781      +/-   ##
==========================================
- Coverage   81.23%   81.02%   -0.21%     
==========================================
  Files         363      365       +2     
  Lines       88404    89067     +663     
==========================================
+ Hits        71811    72167     +356     
- Misses      16593    16900     +307     

Files with missing lines	Coverage Δ
scapy/sendrecv.py	86.53% <100.00%> (+0.11%)	⬆️

... and 41 files with indirect coverage changes

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[gpotter2]

Thanks for the PR. Although I don't understand how you can reach this code (running remains False until you start() ?!), I agree that the current test is broken.

[louis-6wind]

Thank you for the merge. But it is not working for me

Traceback (most recent call last):
  File "/usr/sbin/XXXXX, line 774, in sniffer_close
    sniffer.stop()
  File "/usr/local/lib/python3.12/dist-packages/scapy/sendrecv.py", line 1404, in stop
    if self.continue_sniff:
       ^^^^^^^^^^^^^^^^^^^
AttributeError: 'AsyncSniffer' object has no attribute 'continue_sniff'

[louis-6wind]

To reproduce:

from scapy.all import AsyncSniffer
sniffer=AsyncSniffer(
    iface="ens3",
    store=False,
    promisc=False,
)
sniffer.start()
sniffer.stop()

I got:

Traceback (most recent call last):
File "", line 1, in
File "/usr/local/lib/python3.12/dist-packages/scapy/sendrecv.py", line 1404, in stop
if self.continue_sniff:
^^^^^^^^^^^^^^^^^^^
AttributeError: 'AsyncSniffer' object has no attribute 'continue_sniff'

Basically I am monitoring the interfaces with netlink to start/stop sniffer. It happens in case of flapping