MINOR: log/cbor: _lf_cbor_encode_byte() explicitly requires non-NULL ctx (again)

The BUG_ON() statement that was added in 9bdea51 ("MINOR: log/cbor:
_lf_cbor_encode_byte() explicitly requires non-NULL ctx") isn't
sufficient as Coverity still thinks the lf_buildctx itself may be NULL
as shown in GH #2554. In fact the original reports complains about the
lf_buildctx itself and I didn't understand it properly, let's add another
check in the BUG_ON() to ensure both cbor_ctx and cbor_ctx->ctx are not
NULL since it is not expected if used properly.

src/log.c

@@ -1794,7 +1794,7 @@ static char *_lf_cbor_encode_byte(struct cbor_encode_ctx *cbor_ctx,
 {
 	struct lf_buildctx *ctx;
 
-	BUG_ON(!cbor_ctx);
+	BUG_ON(!cbor_ctx || !cbor_ctx->e_fct_ctx);
 	ctx = cbor_ctx->e_fct_ctx;
 
 	if (ctx->options & LOG_OPT_BIN) {