util/uri: Add overflow check to rfc3986_parse_port

And while at it, replace tabs by eight spaces in this function.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <1424887718-10800-2-git-send-email-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
