MINOR: stktable: avoid ambiguous stktable_data_ptr() usage in cli_io_handler_table()

As reported by @Bbulatov in GH #2586, stktable_data_ptr() return value is
used without checking it isn't NULL first, which may happen if the given
type is invalid or not stored in the table.

However, since date_type is set by table_prepare_data_request() right
before cli_io_handler_table() is invoked, date_type is not expected to
be invalid: table_prepare_data_request() normally checked that the type
is stored inside the table. Thus stktable_data_ptr() should not be failing
at this point, so we add a BUG_ON() to indicate that.

src/stick_table.c

@@ -5597,6 +5597,10 @@ static int cli_io_handler_table(struct appctx *appctx)
 					ptr = stktable_data_ptr(ctx->t,
 								ctx->entry,
 								dt);
+					/* table_prepare_data_request() normally ensures the
+					 * type is both valid and stored
+					 */
+					BUG_ON(!ptr);
 
 					data = 0;
 					switch (stktable_data_types[dt].std_type) {