Ensure that we have a valid witness to IteratorProtocol.next()

Resolves rdar://57003406

Author: CodaFi

lib/Sema/TypeCheckStmt.cpp

@@ -863,6 +863,8 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
 
     auto witness =
         genConformance.getWitnessByName(iteratorTy, getASTContext().Id_next);
+    if (!witness)
+      return nullptr;
     S->setIteratorNext(witness);
 
     auto nextResultType = cast<FuncDecl>(S->getIteratorNext().getDecl())