Errors while ingesting IPFIX

Research, to see, if this is actually a bug and can be marked as triaged.

## Description
Following error is appearing in graylog logs when using IPFIX input with our Stormshield UTM:
```
2020-02-06 14:03:21,878 ERROR: org.graylog2.shared.buffers.processors.DecodingProcessor - Error processing message RawMessage{id=6f35ad37-48e9-11ea-93fc-0242ac120007, journalOffset=3671397671, codec=ipfix, payloadSize=518, timestamp=2020-02-06T14:03:21.859Z, remoteAddress=/192.168.0.254:7804}
java.lang.IndexOutOfBoundsException: readerIndex(150) + length(8) exceeds writerIndex(152): UnpooledHeapByteBuf(ridx: 150, widx: 152, cap: 152/152)
at io.netty.buffer.AbstractByteBuf.checkReadableBytes0(AbstractByteBuf.java:1477) ~[graylog.jar:?]
at io.netty.buffer.AbstractByteBuf.checkReadableBytes(AbstractByteBuf.java:1463) ~[graylog.jar:?]
at io.netty.buffer.AbstractByteBuf.readBytes(AbstractByteBuf.java:896) ~[graylog.jar:?]
at org.graylog.integrations.ipfix.IpfixParser.parseDataSet(IpfixParser.java:364) ~[?:?]
at org.graylog.integrations.ipfix.codecs.IpfixCodec.lambda$decodeMessages$3(IpfixCodec.java:206) ~[?:?]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[?:1.8.0_242]
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382) ~[?:1.8.0_242]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) ~[?:1.8.0_242]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472) ~[?:1.8.0_242]
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[?:1.8.0_242]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:1.8.0_242]
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566) ~[?:1.8.0_242]
at org.graylog.integrations.ipfix.codecs.IpfixCodec.decodeMessages(IpfixCodec.java:212) ~[?:?]
at org.graylog2.shared.buffers.processors.DecodingProcessor.processMessage(DecodingProcessor.java:148) ~[graylog.jar:?]
at org.graylog2.shared.buffers.processors.DecodingProcessor.onEvent(DecodingProcessor.java:91) [graylog.jar:?]
at org.graylog2.shared.buffers.processors.ProcessBufferProcessor.onEvent(ProcessBufferProcessor.java:90) [graylog.jar:?]
at org.graylog2.shared.buffers.processors.ProcessBufferProcessor.onEvent(ProcessBufferProcessor.java:47) [graylog.jar:?]
at com.lmax.disruptor.WorkProcessor.run(WorkProcessor.java:143) [graylog.jar:?]
at com.codahale.metrics.InstrumentedThreadFactory$InstrumentedRunnable.run(InstrumentedThreadFactory.java:66) [graylog.jar:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_242]
```
## Steps To Reproduce

1. Set up an IPFIX input
1. Set up appliance to send IPFIX to the input
1. Look in to graylog logs

## Environment

- Graylog Version: 3.2.1
- Elasticsearch Version: 6.8.6
- MongoDB Version: 3.6.13
- Browser Version: n/a