Fixed chunks being overwritten with empty chunks when the memory setting is immediately changed after a world loads.

src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -799,6 +799,10 @@ public class CraftWorld implements World {
             for (int z = -12; z <= 12; z++) {
                 if (keepLoaded) {
                     loadChunk(chunkCoordX + x, chunkCoordZ + z);
+                } else {
+                    if (isChunkLoaded(chunkCoordX + x, chunkCoordZ + z)) {
+                        if (this.getHandle().getChunkAt(chunkCoordX + x, chunkCoordZ + z).isEmpty()) {
+                            unloadChunk(chunkCoordX + x, chunkCoordZ + z, false);
                         } else {
                             unloadChunk(chunkCoordX + x, chunkCoordZ + z);
                         }
@@ -806,3 +810,5 @@ public class CraftWorld implements World {
                 }
             }
         }
+    }
+}