Make sure to use none backends in generated C code

Otherwise you might run into this:

    Message from VCC-compiler:
    Unused backend nil, defined:
    ('<vcl.inline>' Line 4 Pos 17)
            backend nil none;
    ----------------###------

    (That was just a warning)
    Message from C-compiler:
    vgc.c:1476:20: error: unused variable 'vgc_backend_nil' [-Werror,-Wunused-variable]
    static VCL_BACKEND vgc_backend_nil;
                       ^
    1 error generated.
    Running C-compiler failed, exited with 1
    VCL compilation failed

This is done in both init and discard code to maintain the balance.

bin/varnishtest/tests/v00060.vtc

@@ -43,3 +43,9 @@ client c1 {
 	rxresp
 	expect resp.status == 503
 } -run
+
+varnish v1 -cliok "param.set vcc_err_unref off"
+varnish v1 -vcl {
+	backend bad { .host = "${bad_backend}"; }
+	backend nil none;
+}

lib/libvcc/vcc_backend.c

@@ -330,6 +330,9 @@ vcc_ParseHostDef(struct vcc *tl, const struct token *t_be, const char *vgcname)
 		Fb(tl, 0, "\n\t%s = (NULL);\n", vgcname);
 		vcc_NextToken(tl);
 		SkipToken(tl, ';');
+		ifp = New_IniFin(tl);
+		VSB_printf(ifp->ini, "\t(void)%s;\n", vgcname);
+		VSB_printf(ifp->fin, "\t\t(void)%s;\n", vgcname);
 		return;
 	}