[clang] Fix a null pointer dereference.

[kadircet]

Summary

Sometimes expression inside switch statement can be invalid, for
example type might be incomplete. In those cases code were causing a null
pointer dereference. This patch fixes that.

[sammccall]

Nice!
Can you construct an example and add a simple lit test? rL344133 is a recent similar change.

[kadircet]

• Add tests.

lib/Sema/SemaCodeComplete.cpp

void Sema::CodeCompleteBracketDeclarator(Scope *S) {
   CodeCompleteExpression(S, QualType(getASTContext().getSizeType()));
 }
 
 void Sema::CodeCompleteCase(Scope *S) {
   if (getCurFunction()->SwitchStack.empty() || !CodeCompleter)
     return;
 
   SwitchStmt *Switch = getCurFunction()->SwitchStack.back().getPointer();
+  // Condition expression might be invalid, do not continue in this case.
+  if (!Switch->getCond())
+    return;
   QualType type = Switch->getCond()->IgnoreImplicit()->getType();
   if (!type->isEnumeralType()) {
     CodeCompleteExpressionData Data(type);
     Data.IntegralConstantExpression = true;
     CodeCompleteExpression(S, Data);
     return;
   }