fix: Return correct error in Corpus.Initialize

[MukulKolpe]

The current code incorrectly returns 'err' instead of 'covErr' when handling errors during coverage map updates in Corpus.Initialize. Replaced 'err' with 'covErr' in fuzzing/corpus/corpus.go.

for _, block := range testChain.CommittedBlocks() {
    for _, messageResults := range block.MessageResults {
        covMaps := coverage.GetCoverageTracerResults(messageResults)
        _, _, covErr := c.coverageMaps.Update(covMaps)
        if covErr != nil {
-           return 0, 0, err
+           return 0, 0, covErr
        }
    }
}

Resolves #510

[CLAassistant]

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