CppTools: Remove QTC_ASSERT in completeQtMethodClassName

SOFT ASSERT: "klass" in file src/plugins/cpptools/
cppcompletionassist.cpp, line 1916

...can occur during Qt5 signal/slot completion if the object type can't
be looked up, e.g.:

    Foo *foo; // Foo is used by not defined
    connect(foo, &<TRIGGER COMPLETION HERE>

Change-Id: I0fafd328635e1db57d700d531af0fee819cca8fa
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/cpptools/cppcompletionassist.cpp

@@ -1913,7 +1913,8 @@ bool InternalCppCompletionAssistProcessor::completeQtMethodClassName(
 
     foreach (const LookupItem &lookupItem, results) {
         Class *klass = classFromLookupItem(lookupItem, context);
-        QTC_ASSERT(klass, continue);
+        if (!klass)
+            continue;
         const Name *name = minimalName(klass, cursorScope, context);
         QTC_ASSERT(name, continue);