Clear handle in JBlob when blob is destroyed

src/jrd/EngineInterface.h

@@ -73,6 +73,11 @@ public:
 		return blob;
 	}
 
+	void clearHandle()
+	{
+		blob = NULL;
+	}
+
 private:
 	blb* blob;
 	Firebird::RefPtr<StableAttachmentPart> sAtt;

src/jrd/blb.cpp

@@ -2669,6 +2669,9 @@ void blb::destroy(const bool purge_flag)
 			// in tra_blobs
 			fb_assert(false);
 		}
+
+		if (blb_interface)
+			blb_interface->clearHandle();
 	}
 
 	delete blb_pages;