[analyzer] Fix crash in MoveChecker when it tries to report duplicate issue

The 'MoveChecker' was missing the check if the error node was
successfully generated (non-null value was returned). This happens
if duplicate of the report is emitted.

This patch contains NFC, where 'reportBug' is renamed to 'tryReportBug',
to better indicate conditional behavior of function.

Author: Arseniy Zaostrovnykh <arseniy.zaostrovnykh@sonarsource.com>

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D155084