Running ftools commands such as `join` can clear mata objects unrelated to ftools

[sergiocorreia]

https://github.com/sergiocorreia/ftools/blob/aaeba12507ac38f48b48766a74a124908ba1e62e/src/ftools.mata#L13

Solution: only drop as needed

[bojeryd91]

Hi, I do not understand the solution.

I suspect this issue is why reghdfe clears the mata matrix I'm trying to store results in. I can also report an issue in the reghdfe repository, but I found this while digging. Anyhow, if I execute my code, top to bottom, a matrix created before I run reghdfe is deleted after the run. When I reach the program stops, but if I rerun the same code again, then the matrix is not deleted.