fix NPE with reconnectIfNeeded()

[javeme]

Change-Id: I1e5500ca1c859c2858510100f9f1d5693afdc907
fix: #872

[codecov]

Codecov Report

Merging #874 into master will increase coverage by 1.50%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #874      +/-   ##
============================================
+ Coverage     70.59%   72.10%   +1.50%     
- Complexity     4325     4450     +125     
============================================
  Files           286      287       +1     
  Lines         21084    21205     +121     
  Branches       2969     2994      +25     
============================================
+ Hits          14884    15289     +405     
+ Misses         4710     4411     -299     
- Partials       1490     1505      +15     

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/baidu/hugegraph/api/job/GremlinAPI.java	17.39% <0.00%> (-32.12%)	0.00% <0.00%> (ø%)
...c/main/java/com/baidu/hugegraph/HugeException.java	92.30% <0.00%> (-7.70%)	6.00% <0.00%> (+2.00%)
...aidu/hugegraph/backend/store/hbase/HbaseStore.java	69.68% <0.00%> (-3.69%)	36.00% <0.00%> (+3.00%)
...u/hugegraph/backend/store/hbase/HbaseSessions.java	63.25% <0.00%> (-2.44%)	22.00% <0.00%> (+1.00%)
...idu/hugegraph/job/schema/RebuildIndexCallable.java	84.72% <0.00%> (-2.24%)	21.00% <0.00%> (+1.00%)
.../baidu/hugegraph/license/LicenseVerifyManager.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)	⬆️
...aidu/hugegraph/backend/serializer/BytesBuffer.java	88.88% <0.00%> (ø)	169.00% <0.00%> (ø%)	⬆️
.../hugegraph/backend/store/BackendEntryIterator.java	61.01% <0.00%> (ø)	17.00% <0.00%> (ø%)	⬆️
...egraph/traversal/optimize/HugeScriptTraversal.java	78.04% <0.00%> (ø)	5.00% <0.00%> (ø%)	⬆️
.../main/java/com/baidu/hugegraph/job/GremlinJob.java	95.91% <0.00%> (ø)	8.00% <0.00%> (?%)
... and 28 more

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