searcher: nil pointer panic presence

Created by: slimsag

in Sourcegraph 3.6 users are reporting nil pointer panics. The source of these appears to be here (in the 3.6 branch) in (*readerGrep).Find:

In the above, rg.re can be nil, according to the docstring:

That is called by (*readerGrep).FindZip:

Which is called by concurrentFind:

Which appears to perform no checks about when rg.re is nil, except in a fast-path case:

I don't know under what circumstances we should be checking for rg.re == nil but it is clear we're missing one here.