Clang: Check that follow symbol QFuture is not cancelled

We can't take QFuture result if it's cancelled.

Change-Id: I493e083c0d35e753d1b2616caf2081f472393e34
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangfollowsymbol.cpp

@@ -129,6 +129,8 @@ void ClangFollowSymbol::findLink(const CppTools::CursorInEditor &data,
 
     QObject::connect(&m_watcher, &FutureSymbolWatcher::finished,
                      [=, watcher=&m_watcher, callback=std::move(processLinkCallback)]() mutable {
+        if (watcher->isCanceled())
+            return callback(Utils::Link());
         CppTools::SymbolInfo result = watcher->result();
         // We did not fail but the result is empty
         if (result.fileName.isEmpty()) {