Fix chunk corruption compaction bug.

This fixes part 1) of https://github.com/prometheus/prometheus/issues/367 (the
storing of samples with the wrong fingerprint into a compacted chunk, thus
corrupting it).

Change-Id: I4c36d0d2e508e37a0aba90b8ca2ecc78ee03e3f1

storage/metric/processor.go

@@ -135,6 +135,9 @@ func (p *CompactionProcessor) Apply(sampleIterator leveldb.Iterator, samplesPers
 				return
 			}
 			sampleKey.Load(sampleKeyDto)
+			if !sampleKey.Fingerprint.Equal(fingerprint) {
+				break
+			}
 
 			unactedSamples, err = extractSampleValues(sampleIterator)
 			if err != nil {