mingw: Add warning for falloc

src/option_processing.cc

@@ -337,6 +337,17 @@ error_code::Value option_processing(Option& op, bool standalone,
                 "--save-session");
     op.remove(PREF_DEFERRED_INPUT);
   }
+
+#ifdef __MINGW32__
+  if (op.get(PREF_FILE_ALLOCATION) == V_FALLOC) {
+    A2_LOG_WARN(
+        "--file-allocation=falloc will use SetFileValidData() API, and "
+        "aria2 uses uninitialized disk space which may contain "
+        "confidential data as the download file space. If it is "
+        "undesirable, --file-allocation=prealloc is slower, but safer option.");
+  }
+#endif // __MINGW32__
+
   return error_code::FINISHED;
 }