tell: catch missing message before it causes an exception

[dgw]

Description

This fixes a regression introduced in v7.1.3 by #2162, backported to the 7.1.x branch as 0dec496. (Yep, this one's a mea culpa.)

Previously, the code was sort-of safe due to a strange use of trigger.group(2).lstrip([…]). The new code, using trigger.group(2).split() instead, did not ensure that it actually gets enough values out of the split. I chose instead to make sure enough "words" exist in the command before performing the split.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

Acknowledgements

Thanks to monaco (@hedho) for reporting this via logs on our IRC channel.

[dgw]

Now with lazy tests using @plugin.example. Covering the whole thing seemed like unnecessary work for a bugfix release, and stuff like testing the nickname commands would require either modifying the pytest plugin or creating a whole new suite of command tests in test_module_tell.py. Didn't think either of those was appropriate in a patch intended for backport.