initrd: fix build error with assertions disabled

$ meson -Dmore_asserts=0 meson-build
$ ninja -C meson-build
[712/859] Compiling C object 'src/initrd/b383957@@nmi-core@sta/nmi-cmdline-reader.c.o'.
../src/initrd/nmi-cmdline-reader.c: In function ‘nmi_cmdline_reader_parse’:
../src/initrd/nmi-cmdline-reader.c:871:4: warning: ‘s_ip’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  871 |    nm_setting_ip_config_add_dns (s_ip, ns);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/initrd/nmi-cmdline-reader.c:835:21: note: ‘s_ip’ was declared here
  835 |  NMSettingIPConfig *s_ip;
      |                     ^~~~

Fixes: 25a2b6e14f ('initrd: rework command line parsing')

src/initrd/nmi-cmdline-reader.c

@@ -866,6 +866,7 @@ reader_add_nameservers (Reader *reader, GPtrArray *nameservers)
 				break;
 			default:
 				nm_assert_not_reached ();
+				continue;
 			}
 
 			nm_setting_ip_config_add_dns (s_ip, ns);