ostree: check g_setenv return value

This adds proper return-value checks on g_setenv calls.
It fixes a static analysis warning highlighted by Coverity.

src/libostree/ostree-sepolicy.c

@@ -422,7 +422,10 @@ initable_init (GInitable     *initable,
     {
       const char *policy_rootpath = gs_file_get_path_cached (policy_root);
 
-      g_setenv ("LIBSELINUX_DISABLE_PCRE_PRECOMPILED", "1", FALSE);
+      /* TODO(lucab): get rid of this setenv(), it may be unsafe in a multi-thread context. */
+      if (!g_setenv ("LIBSELINUX_DISABLE_PCRE_PRECOMPILED", "1", FALSE))
+        return glnx_throw (error, "Failed to set environment variable LIBSELINUX_DISABLE_PCRE_PRECOMPILED");
+
       if (selinux_set_policy_root (policy_rootpath) != 0)
         return glnx_throw_errno_prefix (error, "selinux_set_policy_root(%s)", policy_rootpath);