MINOR: Call logSegments.toBuffer only when required (#9971)

Reviewers: Ismael Juma <ismael@juma.me.uk>, Satish Duggana <satishd@apache.org>, Chia-Ping Tsai <chia7712@gmail.com>

core/src/main/scala/kafka/log/Log.scala

@@ -1648,9 +1648,6 @@ class Log(@volatile private var _dir: File,
           s"for partition $topicPartition is ${config.messageFormatVersion} which is earlier than the minimum " +
           s"required version $KAFKA_0_10_0_IV0")
 
-      // Cache to avoid race conditions. `toBuffer` is faster than most alternatives and provides
-      // constant time access while being safe to use with concurrent collections unlike `toArray`.
-      val segmentsCopy = logSegments.toBuffer
       // For the earliest and latest, we do not need to return the timestamp.
       if (targetTimestamp == ListOffsetsRequest.EARLIEST_TIMESTAMP) {
         // The first cached epoch usually corresponds to the log start offset, but we have to verify this since
@@ -1667,6 +1664,9 @@ class Log(@volatile private var _dir: File,
         val epochOptional = Optional.ofNullable(latestEpochOpt.orNull)
         Some(new TimestampAndOffset(RecordBatch.NO_TIMESTAMP, logEndOffset, epochOptional))
       } else {
+        // Cache to avoid race conditions. `toBuffer` is faster than most alternatives and provides
+        // constant time access while being safe to use with concurrent collections unlike `toArray`.
+        val segmentsCopy = logSegments.toBuffer
         // We need to search the first segment whose largest timestamp is >= the target timestamp if there is one.
         val targetSeg = segmentsCopy.find(_.largestTimestamp >= targetTimestamp)
         targetSeg.flatMap(_.findOffsetByTimestamp(targetTimestamp, logStartOffset))