libxl: Fix NULL deref in libxl_ctx_free()

xc_interface_close() will de-reference a NULL handle, check for this
condition in libxl_ctx_free() before calling.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl.c

@@ -67,7 +67,7 @@ int libxl_ctx_init(libxl_ctx *ctx, int version, xentoollog_logger *lg)
 
 int libxl_ctx_free(libxl_ctx *ctx)
 {
-    xc_interface_close(ctx->xch);
+    if (ctx->xch) xc_interface_close(ctx->xch);
     libxl_version_info_destroy(&ctx->version_info);
     if (ctx->xsh) xs_daemon_close(ctx->xsh); 
     return 0;