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.

src/stick_table.c

@@ -5025,7 +5025,7 @@ static int table_process_entry(struct appctx *appctx, struct stksess *ts, char *
 				return 1;
 			}
 
-			ptr = stktable_data_ptr(t, ts, data_type);
+			ptr = __stktable_data_ptr(t, ts, data_type);
 
 			switch (stktable_data_types[data_type].std_type) {
 			case STD_T_SINT: