[xen] Use standard calling pattern for asprintf()

Our asprintf() implementation guarantees that strp will be NULL on
allocation failure, but this is not standard behaviour.  Detect errors
by checking for a negative return value instead of a NULL pointer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/xen/xenstore.c

 		      child += ( strlen ( child ) + 1 /* NUL */ ) ) {
 
 			/* Construct child key */
-			asprintf ( &child_key, "%s/%s", key, child );
-			if ( ! child_key ) {
+			if ( asprintf ( &child_key, "%s/%s", key, child ) < 0 ){
 				DBGC ( xen, "XENSTORE could not allocate child "
 				       "key \"%s/%s\"\n", key, child );
 				rc = -ENOMEM;