Fixed an issue with strlen checking null ptr

Program called "Moom" (http://manytricks.com/moom/) which uses OS X's
accesibility API could cause a crash with calling the log handler with a
null file in context.

Author: obscuren

cpp/capi.cpp

@@ -855,6 +855,8 @@ QQmlListProperty_ *newListProperty(GoAddr *addr, intptr_t reflectIndex, intptr_t
 
 void internalLogHandler(QtMsgType severity, const QMessageLogContext &context, const QString &text)
 {
+    if (context.file == NULL) return;
+
     QByteArray textba = text.toUtf8();
     LogMessage message = {severity, textba.constData(), textba.size(), context.file, (int)strlen(context.file), context.line};
     hookLogHandler(&message);