vlan plugin - fix invalid return code from fill_ipfix()

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0f52d43) 41.31% compared to head (0b3820a) 41.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #190 +/- ## ======================================= Coverage 41.31% 41.31% ======================================= Files 94 94 Lines 8660 8660 Branches 1299 1299 ======================================= Hits 3578 3578 Misses 4314 4314 Partials 768 768 ``` | [Flag](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/190/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/190/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | `41.31% <0.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | Coverage Δ | | |---|---|---| | [process/vlan.hpp](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#diff-cHJvY2Vzcy92bGFuLmhwcA==) | `30.00% <0.00%> (ø)` | |

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