bugfix for JSONValidator, fix #3293

[ZivYan]

处理当 supportMultiValue 为 true 的时候并且 json 只为 single，则永远会为 false，所以将 eof 判断移动到判断 supportMultiValue 之前，解决该问题

[codecov-commenter]

Codecov Report

Merging #3296 into master will increase coverage by 0.10%.
The diff coverage is 57.14%.

@@             Coverage Diff              @@
##             master    #3296      +/-   ##
============================================
+ Coverage     79.92%   80.02%   +0.10%     
- Complexity     7381     7392      +11     
============================================
  Files           157      157              
  Lines         27778    27784       +6     
  Branches       6678     6681       +3     
============================================
+ Hits          22201    22234      +33     
+ Misses         3591     3558      -33     
- Partials       1986     1992       +6     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/com/alibaba/fastjson/JSONObject.java	84.37% <0.00%> (-1.06%)	95.00 <0.00> (ø)
.../main/java/com/alibaba/fastjson/JSONValidator.java	24.10% <66.66%> (+11.41%)	20.00 <0.00> (+8.00)
...main/java/com/alibaba/fastjson/util/TypeUtils.java	79.93% <80.00%> (ø)	646.00 <0.00> (+2.00)
...ava/com/alibaba/fastjson/serializer/DateCodec.java	92.38% <100.00%> (+0.07%)	50.00 <0.00> (+2.00)
...com/alibaba/fastjson/serializer/SerialContext.java	82.69% <0.00%> (-3.85%)	21.00% <0.00%> (-1.00%)
...main/java/com/alibaba/fastjson/util/FieldInfo.java	86.44% <0.00%> (ø)	93.00% <0.00%> (-1.00%)
...com/alibaba/fastjson/parser/JSONReaderScanner.java	74.84% <0.00%> (+1.25%)	46.00% <0.00%> (+1.00%)

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