Fix assertion in ReadOnlyEngine (#43010)

We should execute the assertion before throwing an exception; 
otherwise, it's a noop.

Author: dnhatn

server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java

@@ -141,12 +141,12 @@ protected void ensureMaxSeqNoEqualsToGlobalCheckpoint(final SeqNoStats seqNoStat
         final Version indexVersionCreated = engineConfig.getIndexSettings().getIndexVersionCreated();
         if (indexVersionCreated.onOrAfter(Version.V_7_2_0) ||
             (seqNoStats.getGlobalCheckpoint() != SequenceNumbers.UNASSIGNED_SEQ_NO)) {
+            assert assertMaxSeqNoEqualsToGlobalCheckpoint(seqNoStats.getMaxSeqNo(), seqNoStats.getGlobalCheckpoint());
             if (seqNoStats.getMaxSeqNo() != seqNoStats.getGlobalCheckpoint()) {
                 throw new IllegalStateException("Maximum sequence number [" + seqNoStats.getMaxSeqNo()
                     + "] from last commit does not match global checkpoint [" + seqNoStats.getGlobalCheckpoint() + "]");
             }
         }
-        assert assertMaxSeqNoEqualsToGlobalCheckpoint(seqNoStats.getMaxSeqNo(), seqNoStats.getGlobalCheckpoint());
     }
 
     protected boolean assertMaxSeqNoEqualsToGlobalCheckpoint(final long maxSeqNo, final long globalCheckpoint) {