Avoid to wipe a user buffer when writing candidates.

[mookid]

Fix #1772.

I was able to reproduce the issue with counsel-ag, and I suspect that the bug impacts any usage of counsel--async-command as well.

My guess is that counsel--async-filter is a caller of ivy--insert-minibuffer, which must only be called when the current buffer is the minibuffer (ie, ivy--insert-minibuffer is not wrapped in a with-current-buffer block).

When called with a process that exists immediately, it is very unlikely that the process filter will be called at all, and even less before the user had time to change buffer himself.

@jojojames can you confirm that this version fixes the issue?

[jojojames]

@mookid Thanks for the fix!

I'll be a little busy for the next few weeks to a month but I'll see if I can squeeze in some time to test this.

Thanks.

[abo-abo]

Thanks.