vrt_ctx: Make ctx->vsl not NULL in vcl_{init,fini}

This is a common source of panic when vmods functions/methods
logging to vsl get called from housekeeping vcl subroutines.

Refs #2998

Author: walid-git

bin/varnishd/cache/cache_vcl.c

@@ -136,9 +136,8 @@ VCL_Get_CliCtx(int msg)
 	if (msg) {
 		ctx_cli.msg = VSB_new_auto();
 		AN(ctx_cli.msg);
-	} else {
-		ctx_cli.vsl = &vsl_cli;
 	}
+	ctx_cli.vsl = &vsl_cli;
 	ctx_cli.ws = &ws_cli;
 	WS_Assert(ctx_cli.ws);
 	return (&ctx_cli);

bin/varnishd/cache/cache_vrt.c

@@ -683,12 +683,12 @@ VRT_fail(VRT_CTX, const char *fmt, ...)
 	AN(fmt);
 	AZ(strchr(fmt, '\n'));
 	va_start(ap, fmt);
-	if (ctx->vsl != NULL) {
-		VSLbv(ctx->vsl, SLT_VCL_Error, fmt, ap);
-	} else {
-		AN(ctx->msg);
+	if (ctx->msg != NULL) {
 		VSB_vprintf(ctx->msg, fmt, ap);
 		VSB_putc(ctx->msg, '\n');
+	} else {
+		AN(ctx->vsl);
+		VSLbv(ctx->vsl, SLT_VCL_Error, fmt, ap);
 	}
 	va_end(ap);
 	ctx->vpi->handling = VCL_RET_FAIL;