Add null check before accessing packetWriteMap

[joeygrover]

Fixes #951

This PR is ready for review.

Risk

This PR makes no API changes.

Summary

• Added a null check before accessing offending variable

Changelog

Bug Fixes

• Prevent NPE during a router service shutdown

CLA

• I have signed the CLA

[codecov-io]

Codecov Report

Merging #957 into develop will decrease coverage by <.01%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##             develop     #957      +/-   ##
=============================================
- Coverage      42.55%   42.55%   -0.01%     
  Complexity      3735     3735              
=============================================
  Files            434      434              
  Lines          23098    23099       +1     
  Branches        2698     2699       +1     
=============================================
  Hits            9829     9829              
- Misses         12623    12624       +1     
  Partials         646      646

Impacted Files	Coverage Δ	Complexity Δ
...om/smartdevicelink/transport/SdlRouterService.java	7.19% <0%> (-0.01%)	9 <0> (ø)

---

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 7d329a1...0f3746a. Read the comment docs.