libnm: fix wrong variable in route attribute validation error message

The validation checks 'addr' (the extracted address portion before '/')
but the error message prints 'string' (the full input including the
prefix). For input like "192.168.1.999/24", the error would show the
full string instead of just the invalid address part.

Found by Coverity (CID: COPY_PASTE_ERROR).

Fixes: 539db43619 ('libnm: avoid heap allocation for checking valid routes in nm_ip_route_attribute_validate()')

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
(cherry picked from commit 3d4ad7b4ba)

src/libnm-core-impl/nm-setting-ip-config.c

@@ -1401,7 +1401,7 @@ _ip_route_attribute_validate(const char           *name,
                         NM_CONNECTION_ERROR_FAILED,
                         family == AF_INET ? _("'%s' is not a valid IPv4 address")
                                           : _("'%s' is not a valid IPv6 address"),
-                        string);
+                        addr);
             return FALSE;
         }
         break;