RDKBDEV-3364:Fix memory leak found by coverity in WanManager

[owen-lu-sercomm]

Reason for change:
By scanning the code with Coverity, we identified a risk of memory leak in the WanManager, so added fix to avoid the memory leak.
Test Procedure:

1. scanning the code with Coverity, to check if the memory leak is still reported
2. plug in the wan cable and let wan up, check if the wan basic can work normally
   Risks: Low

Signed-off-by: Owen Lu owen_lu@sercomm.com

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