remote.c: correct the check for a leading '/' in a remote name

This test is supposed to disallow remote entries in the config file of the
form:

   [remote "/foobar"]
      ...

The leading slash in '/foobar' is not acceptable.

Instead it was incorrectly testing that the subkey had no leading '/', which
had no effect since the subkey pointer was made to point at a '.' in the
preceding lines.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Acked-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>