win32: clear file dialog string before triggering callback

Things would go weird if a plugin opens a new file dialog in the
file callback, prevent this by clearing the string before the callback.

dgl/src/Window.cpp

@@ -780,8 +780,9 @@ struct Window::PrivateData {
 #if defined(DISTRHO_OS_WINDOWS) && !defined(DGL_FILE_BROWSER_DISABLED)
         if (fSelectedFile.isNotEmpty())
         {
-            fView->fileSelectedFunc(fView, fSelectedFile.buffer());
-            fSelectedFile.clear();
+            char* const buffer = fSelectedFile.getAndReleaseBuffer();
+            fView->fileSelectedFunc(fView, buffer);
+            std::free(buffer);
         }
 #endif
 

distrho/extra/String.hpp

@@ -568,6 +568,18 @@ public:
         return fBuffer;
     }
 
+    /*
+     * Get and release the string buffer, while also clearing this string.
+     * Result must be freed.
+     */
+    char* getAndReleaseBuffer() noexcept
+    {
+        char* const ret = fBuffer;
+        fBuffer = _null();
+        fBufferLen = 0;
+        return ret;
+    }
+
     // -------------------------------------------------------------------
     // base64 stuff, based on http://www.adp-gmbh.ch/cpp/common/base64.html
     // Copyright (C) 2004-2008 René Nyffenegger