Increased VACUUM safety margin

app/src/main/java/eu/faircode/email/WorkerCleanup.java

@@ -229,12 +229,17 @@ public class WorkerCleanup extends Worker {
             if (manual) {
                 // https://www.sqlite.org/lang_vacuum.html
                 long size = context.getDatabasePath(db.getOpenHelper().getDatabaseName()).length();
-                long space = Helper.getAvailableStorageSpace();
-                if (size * 2 < space) {
-                    Log.i("Running VACUUM");
+                long available = Helper.getAvailableStorageSpace();
+                if (size > 0 && size * 2.5 < available) {
+                    Log.i("Running VACUUM" +
+                            " size=" + Helper.humanReadableByteCount(size, true) +
+                            "/" + Helper.humanReadableByteCount(available, true));
                     db.getOpenHelper().getWritableDatabase().execSQL("VACUUM;");
                 } else
-                    Log.w("Insufficient space for VACUUM");
+                    Log.w("Insufficient space for VACUUM" +
+                            " size=" + Helper.humanReadableByteCount(size, true) +
+                            "/" + Helper.humanReadableByteCount(available, true));
+
             }
 
         } catch (Throwable ex) {