Don't crash when the /proc/<pid>/stat file failed to open.

This may happen if a process exits after we got the /proc listing. If
we fail to open the file, readAll() returns empty, which means split()
returns an empty list, which means you can't do at(1).

src/plugins/debugger/debuggerdialogs.cpp

@@ -197,7 +197,9 @@ static QList<ProcData> unixProcessList()
         filename += procId;
         filename += QLatin1String("/stat");
         QFile file(filename);
-        file.open(QIODevice::ReadOnly);
+        if (!file.open(QIODevice::ReadOnly))
+            continue;           // process may have exited
+
         const QStringList data = QString::fromLocal8Bit(file.readAll()).split(' ');
         ProcData proc;
         proc.ppid = procId;