content_error(): Don't strlen(invo_name) that might be NULL.

content_error(): Don't strlen(invo_name) that might be NULL.

Earlier in content_error(), it checks invo_name isn't NULL before using
it.  Later, it passes it to strlen(3) without the check.  Either the
former is redundant or the latter wrong.  Rather than work out which,
delete the strlen(invo_name) because it was only used for a
variable-width indent on a second diagnostic line.  Instead, indent by a
fixed four spaces, which will look better when invo_name is stupidly
long anyway.  Bug present since pre-git.