Only print stdout buf of runner if it exists

This fixes a crash that happens when quitting fzz with Ctrl-D or return
right after starting it. The problem was that quitting would result in
runner trying to send the buffered stdout of the last command to the
printer, but since no command has been run, the stdout buf does not yet
exist.

This checks if it exists and only prints then, otherwise it silently
returns.
Validate placeholder when starting up

Since we can set a placeholder with the environment variable
FZZ_PLACEHOLDER instead of using the default placeholder we now validate
this custom placeholder.

The plan is to allow users to specify initial input with the
placeholder, like this:

  fzz ag {{foobar}} *.go

In order to to that the number of characters in the placeholder has to
be valid.