Close connection after error on open (#351)

As per [documentation](https://www.sqlite.org/c3ref/open.html), even when the
database opening failed, we should still run `sqlite3_close()` to free the
resources:

> Whether or not an error occurs when it is opened, resources associated with
> the [database connection](https://www.sqlite.org/c3ref/sqlite3.html) handle
> should be released by passing it to [sqlite3_close()](https://www.sqlite.org/c3ref/close.html)
> when it is no longer required.

I verified this against Python implementation, which
[does this too](https://github.com/python/cpython/blob/357c6500589ca7e065a6c263accfa1307d93c990/Modules/_sqlite/connection.c#L334-L339).
Python also calls `assert` to make sure that closing was clean (it should be per
a comment there), but I think it's not a good idea for a NIF to do such thing.

Author: katafrakt

c_src/sqlite3_nif.c

@@ -461,6 +461,7 @@ exqlite_open(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
 
     rc = sqlite3_open_v2((char*)bin.data, &db, flags, NULL);
     if (rc != SQLITE_OK) {
+        sqlite3_close_v2(db);
         return make_error_tuple(env, am_database_open_failed);
     }