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/bitcoin.cpp

@@ -578,7 +578,8 @@ int main(int argc, char *argv[])
 
     /// 5. Now that settings and translations are available, ask user for data directory
     // User language is set up: pick a data directory
-    Intro::pickDataDirectory();
+    if (!Intro::pickDataDirectory())
+        return 0;
 
     /// 6. Determine availability of data directory and parse bitcoin.conf
     /// - Do not call GetDataDir(true) before this step finishes