Fix log rotation

See https://github.com/profanity-im/profanity/issues/1518
It has a good explanation of what happened:

```
Apparently, the _rotate_log_file function tried to extract user-provided
name from currently used mainlogfile and restart logging to the same
place after rotation, but currently this is interpreted as a full path
instead. As I understand, the log rotation is no longer done with
user-provided paths at all so this should be simply skipped altogether
now as passing any non-NULL value is interpreted as user-provided.
Replacing start with NULL appears to fix it for me.
```

In log_msg() we only rotate the log if not user_provided_log.

https://github.com/profanity-im/profanity/pull/1455 changed the
behaviour from user defined filename in the log dir to using full path.

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