fix some index check for decode

[leizhiyuan]

Ⅰ. Describe what this PR did

fix some logic bug when decode

Ⅱ. Does this pull request fix one issue?

fixes #909

Ⅲ. Why don't you add test cases (unit test/integration test)?

please review it， there some scenes， which member cloud be null，such as extraData，maybe I misunderstand it。

in this pr，I do not fix the merged message decoder

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #916 into develop will decrease coverage by 0.19%.
The diff coverage is 20.49%.

@@             Coverage Diff              @@
##             develop     #916     +/-   ##
============================================
- Coverage      37.17%   36.97%   -0.2%     
  Complexity      1033     1033             
============================================
  Files            225      225             
  Lines           8923     8908     -15     
  Branches        1079     1080      +1     
============================================
- Hits            3317     3294     -23     
+ Misses          5215     5212      -3     
- Partials         391      402     +11

Impacted Files	Coverage Δ	Complexity Δ
...o/seata/core/protocol/AbstractIdentifyRequest.java	47.31% <0%> (-2.69%)	12 <0> (ø)
...java/io/seata/core/protocol/MergedWarpMessage.java	72.72% <0%> (-1.19%)	8 <1> (ø)
...a/io/seata/core/rpc/netty/MessageCodecHandler.java	0% <0%> (ø)	0 <0> (ø)	⬇️
.../seata/core/protocol/AbstractIdentifyResponse.java	45.83% <0%> (+0.93%)	4 <0> (ø)	⬇️
...ava/io/seata/core/protocol/MergeResultMessage.java	76.19% <0%> (-1.09%)	10 <1> (ø)
.../io/seata/core/protocol/AbstractResultMessage.java	66.03% <0%> (-0.04%)	14 <1> (ø)
...rotocol/transaction/AbstractBranchEndResponse.java	65.62% <33.33%> (-8.06%)	10 <0> (ø)
...java/io/seata/core/protocol/RegisterRMRequest.java	56.06% <33.33%> (-14.09%)	8 <1> (ø)
...protocol/transaction/AbstractBranchEndRequest.java	68.31% <41.93%> (-10.04%)	14 <1> (ø)

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[zhangthen]

Okey to me.

[wgs13579]

LGTM