修复hessian在集合类反序列化时，包装类为null返回基本类型默认值的bug

[fingthinking]

修复hessian在集合类反序列化时，包装类为null返回基本类型默认值的bug: apache/dubbo#3640
之前看到合入的某个bugfix: #8，会出现多线程问题，而且不是从本质上解决类型转换错误的问题。

[codecov-io]

Codecov Report

Merging #11 into master will increase coverage by 0.21%.
The diff coverage is 75%.

@@             Coverage Diff              @@
##             master      #11      +/-   ##
============================================
+ Coverage     24.46%   24.68%   +0.21%     
- Complexity      460      471      +11     
============================================
  Files            89       89              
  Lines          6302     6299       -3     
  Branches       1048     1050       +2     
============================================
+ Hits           1542     1555      +13     
+ Misses         4545     4524      -21     
- Partials        215      220       +5

Impacted Files	Coverage Δ	Complexity Δ
...ba/com/caucho/hessian/io/AbstractHessianInput.java	7.69% <ø> (ø)	1 <0> (ø)	⬇️
...om/alibaba/com/caucho/hessian/io/HessianInput.java	19.34% <0%> (-0.04%)	46 <1> (+1)
...ibaba/com/caucho/hessian/io/BasicDeserializer.java	6.99% <100%> (+1.6%)	11 <8> (+3)	⬆️
.../com/caucho/hessian/io/CollectionDeserializer.java	67.39% <50%> (-5.69%)	12 <0> (-2)
...m/alibaba/com/caucho/hessian/io/Hessian2Input.java	19.93% <85.71%> (+0.23%)	67 <2> (+3)	⬆️
.../alibaba/com/caucho/hessian/io/Hessian2Output.java	25.12% <0%> (+0.69%)	35% <0%> (+2%)	⬆️
...alibaba/com/caucho/hessian/io/BasicSerializer.java	17.39% <0%> (+4.34%)	7% <0%> (+2%)	⬆️
...libaba/com/caucho/hessian/util/IdentityIntMap.java	39.28% <0%> (+7.14%)	9% <0%> (+2%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5a74492...1110eb4. Read the comment docs.

[CrazyHZM]

my fault，
This PR looks good.

[jonathenzc]

这里的tag有可能返回-1，在_buffer[_offset++]中会出现ArrayIndexOutOfBoundsException的问题。可在下面offset--处做个保护