fuzzing: fixed calculation of serializedSize() in features.go
fuzzing: fixed calculation of serializedSize() in features.go

This commit fixes an incorrectly calculated size of a
*FeatureVector in the serializedSize() function. go-fuzz
found that when calling NewFeatureVectorFromReader, if
a flag is invalid, it is not added to f.flags. However,
it will skip the index that wasn't included. This
becomes a problem when serializedSize() calculates the
length of f.flags via len() which can lead to an index
out of range since certain flags may be missing.

LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd