submodule_init: die cleanly on submodules without url defined

submodule_init: die cleanly on submodules without url defined

When we init a submodule, we try to die when it has no URL
defined:

  url = xstrdup(sub->url);
  if (!url)
	  die(...);

But that's clearly nonsense. xstrdup() will never return
NULL, and if sub->url is NULL, we'll segfault.

These two bits of code need to be flipped, so we check
sub->url before looking at it.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>