watch_csv() plugin crashes darwin client in 0.6.8 and newer

[SBattaglia-R7]

Possibly related to #2450 . I've only verified the crash on MacOS since I don't have access to Linux or Windows right now.

I tested on 0.6.7, 0.6.8, 0.6.9, and 0.7.0 and verified that the file path is correct by using parse_csv() with the same parameters. Parse_csv() outputs the expected file contents and does not crash on the tested versions.

To reproduce, query "SELECT * from watch_csv(filename='path/to/file')". On 0.6.8 and above, the client will panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x5316f72]

goroutine 122 [running]:
www.velocidex.com/golang/velociraptor/vql/parsers/csv.(*CSVWatcherService).Register(0x0, 0xc0003fde40, {0x0, 0x0}, {0x5fe0898?, 0xc000442300}, {0x5ff19c0, 0xc0012ae480}, 0xc000735080)
	/Users/mic/projects/velociraptor/vql/parsers/csv/watcher.go:55 +0x92
www.velocidex.com/golang/velociraptor/vql/parsers/csv._WatchCSVPlugin.Call.func1()
	/Users/mic/projects/velociraptor/vql/parsers/csv/csv.go:195 +0x318
created by www.velocidex.com/golang/velociraptor/vql/parsers/csv._WatchCSVPlugin.Call
	/Users/mic/projects/velociraptor/vql/parsers/csv/csv.go:174 +0xf8

The panic occurs whether the file path is correct or not. On 0.6.7, the client does not crash, but it also does not fire events when a file in the provided path is edited.

[scudette]

Fixed by #2901