qt: Fix random segfault when closing "Choose data directory" dialog

The `pickDataDirectory()` function was calling `exit(0)` to quit
the application when the user closes the dialog without choosing
a data directory.

This is a bad idea because a background thread is created (to
check free space on the drive of the currently selected datadir).
The thread is not stopped and unwound properly, resulting in a potential
race condition somewhere deep in Qt.

So replace the `exit()` by a boolean return value, and let the
stack unwind normally.

src/qt/intro.cpp

@@ -165,14 +165,14 @@ QString Intro::getDefaultDataDirectory()
     return GUIUtil::boostPathToQString(GetDefaultDataDir());
 }
 
-void Intro::pickDataDirectory()
+bool Intro::pickDataDirectory()
 {
     namespace fs = boost::filesystem;
     QSettings settings;
     /* If data directory provided on command line, no need to look at settings
        or show a picking dialog */
     if(!GetArg("-datadir", "").empty())
-        return;
+        return true;
     /* 1) Default data directory for operating system */
     QString dataDir = getDefaultDataDirectory();
     /* 2) Allow QSettings to override default dir */
@@ -190,7 +190,7 @@ void Intro::pickDataDirectory()
             if(!intro.exec())
             {
                 /* Cancel clicked */
-                exit(0);
+                return false;
             }
             dataDir = intro.getDataDirectory();
             try {
@@ -211,6 +211,7 @@ void Intro::pickDataDirectory()
      */
     if(dataDir != getDefaultDataDirectory())
         SoftSetArg("-datadir", GUIUtil::qstringToBoostPath(dataDir).string()); // use OS locale for path setting
+    return true;
 }
 
 void Intro::setStatus(int status, const QString &message, quint64 bytesAvailable)