Fix detected defects from static analysis

:Release Notes:
Fix detected defects from static analysis for zero bugs

:Detailed Notes:
- MojString::assign() without checking return value;
- MojDbShardManagerTest::testShardEngine(): unchecked return value.

:Testing Performed:
tested on build

:QA Notes:

:Issues Addressed:
[GF-54270] Coverity issues: db8

Open-webOS-DCO-1.0-Signed-off-by: Igor Kuksyuk <igor.kuksiuk@lge.com>

Change-Id: Ia64dfb2f5314712d185b4f2b4c87de98506f89d8
Reviewed-on: https://g2g.palm.com/4158
Reviewed-by: DCO Verification
Reviewed-by: Igor Kuksyuk <igor.kuksiuk@lge.com>
Tested-by: Igor Kuksyuk <igor.kuksiuk@lge.com>
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Tested-by: Maksym Sditanov <maxim.sditanov@lge.com>