MINOR: stktable: stktable_data_ptr() cannot fail in table_process_entry()

MINOR: stktable: stktable_data_ptr() cannot fail in table_process_entry()

In table_process_entry(), stktable_data_ptr() result is dereferenced
without checking if it's NULL first, which may happen when bad inputs
are provided to the function.

However, data_type and ts arguments were already checked prior to calling
the function, so we know for sure that stktable_data_ptr() will never
return NULL in this case.

However some static code analyzers such as Coverity are being confused
because they think that the result might possibly be NULL.
(See GH #2398)

To make it explicit that we always provide good inputs and expect valid
result, let's switch to the __stktable_data_ptr() unsafe function.