libxl: fix typo in libxl__hotplug_nic error checking

Coverity-ID: 1055945
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/libxl_linux.c

@@ -170,7 +170,7 @@ static int libxl__hotplug_nic(libxl__gc *gc, libxl__device *dev,
     }
 
     *env = get_hotplug_env(gc, script, dev);
-    if (!env) {
+    if (!*env) {
         rc = ERROR_FAIL;
         goto out;
     }