checkout: do not corrupt HEAD on empty repo

checkout: do not corrupt HEAD on empty repo

In abe1998 ("git checkout -b: allow switching out of an unborn
branch"), a code-path overly-optimisticly assumed that a
branch-name was specified. This is not always the case, and as
a result a NULL-pointer was attempted printed to .git/HEAD.

This could lead to at least two different failure modes:
 1) vsnprintf formated the NULL-string as something useful (e.g
    "(null)")
 2) vsnprintf crashed

Neither were very convenient for formatting a new HEAD-reference.

To fix this, reintroduce some strictness so we only take this
new codepath if a banch-name was specified.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>