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

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.