Fixed the bug due to dangling pointers in RequestGroup.

RequestGroup holds a poitner to btRuntime_ and peerStorage_. After
removing them from BtRegistry, we failed to set 0 to them.  When
program access them, it goes undefined world, such as random crash.
We found this bug when pasuing download and valgrind warned memory
corruption.

src/RequestGroup.cc

@@ -1021,6 +1021,8 @@ void RequestGroup::releaseRuntimeResource(DownloadEngine* e)
 {
 #ifdef ENABLE_BITTORRENT
   e->getBtRegistry()->remove(gid_);
+  btRuntime_ = 0;
+  peerStorage_ = 0;
 #endif // ENABLE_BITTORRENT
   if(pieceStorage_) {
     pieceStorage_->removeAdvertisedPiece(0);