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

[tomasz-kaminski-sonarsource]

Summary

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 <[e-mail]>

clang/lib/StaticAnalyzer/Checkers/MoveChecker.cpp

private:
   // this function was called should immediately return after the call
   // because this function adds one or two transitions.
   void modelUse(ProgramStateRef State, const MemRegion *Region,
                 const CXXRecordDecl *RD, MisuseKind MK,
                 CheckerContext &C) const;
 
   // Returns the exploded node against which the report was emitted.
   // The caller *must* add any further transitions against this node.
-  ExplodedNode *reportBug(const MemRegion *Region, const CXXRecordDecl *RD,
+  // Returns nullptr and does not report if such node already exists.
+  ExplodedNode *tryToReportBug(const MemRegion *Region, const CXXRecordDecl *RD,
                                CheckerContext &C, MisuseKind MK) const;
 
   bool isInMoveSafeContext(const LocationContext *LC) const;
   bool isStateResetMethod(const CXXMethodDecl *MethodDec) const;
   bool isMoveSafeMethod(const CXXMethodDecl *MethodDec) const;
   const ExplodedNode *getMoveLocation(const ExplodedNode *N,
                                       const MemRegion *Region,
                                       CheckerContext &C) const;
 };
if (isAnyBaseRegionReported(State, Region)) {
     if (misuseCausesCrash(MK)) {
       C.generateSink(State, C.getPredecessor());
     } else {
       C.addTransition(State);
     }
     return;
   }
 
-  ExplodedNode *N = reportBug(Region, RD, C, MK);
+  ExplodedNode *N = tryToReportBug(Region, RD, C, MK);
 
   // If the program has already crashed on this path, don't bother.
-  if (N->isSink())
+  if (!N || N->isSink())
     return;
 
   State = State->set<TrackedRegionMap>(Region, RegionState::getReported());
   C.addTransition(State, N);
 }
 
-ExplodedNode *MoveChecker::reportBug(const MemRegion *Region,
-                                     const CXXRecordDecl *RD, CheckerContext &C,
+ExplodedNode *MoveChecker::tryToReportBug(const MemRegion *Region,
+                                          const CXXRecordDecl *RD,
+                                          CheckerContext &C,
                                           MisuseKind MK) const {
   if (ExplodedNode *N = misuseCausesCrash(MK) ? C.generateErrorNode()
                                               : C.generateNonFatalErrorNode()) {
     // Uniqueing report to the same object.
     PathDiagnosticLocation LocUsedForUniqueing;
     const ExplodedNode *MoveNode = getMoveLocation(N, Region, C);
 
     if (const Stmt *MoveStmt = MoveNode->getStmtForDiagnostics())
       LocUsedForUniqueing = PathDiagnosticLocation::createBegin(