Open_DB: Initialize totlen

Variable 'totlen' is uninitialized when used in 'Open_DB'.

See https://github.com/thegenemyers/DALIGNER/blob/master/DB.c#L375
and https://github.com/thegenemyers/DALIGNER/blob/master/DB.c#L385

```c
      int64      totlen;
…
          totlen += r;
```

```
$ clang -O4 -Wall -Wextra   -c -o DB.o DB.c
clang: warning: -O4 is equivalent to -O3
DB.c:385:11: warning: variable 'totlen' is uninitialized when used here
      [-Wuninitialized]
          totlen += r;
          ^~~~~~
DB.c:375:24: note: initialize the variable 'totlen' to silence this warning
      int64      totlen;
                       ^
                        = 0
1 warning generated.
```