Don't continue trying to deserialize blank block data

src/database-leveldb.cpp

@@ -104,6 +104,7 @@ MapBlock* Database_LevelDB::loadBlock(v3s16 blockpos)
 		} else {
 			throw SerializationError("Blank block data in database");
 		}
+		return NULL;
 	}
 
 	if (s.ok()) {

src/database-sqlite3.cpp

@@ -237,6 +237,7 @@ MapBlock* Database_SQLite3::loadBlock(v3s16 blockpos)
 			} else {
 				throw SerializationError("Blank block data in database");
 			}
+			return NULL;
 		}
 
 		std::string datastr(data, len);