clang: suppress a warning

Initialize grn_id explicitly.

    db.c:2430:7: warning: variable 'id' is used uninitialized whenever 'if'
          condition is false [-Wsometimes-uninitialized]
          WITH_NORMALIZE(hash, key, key_size, {
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    db.c:46:9: note: expanded from macro 'WITH_NORMALIZE'
        if ((nstr = grn_string_open(ctx, key, key_size,\
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    db.c:2433:11: note: uninitialized use occurs here
          if (id) { grn_table_add(ctx, res, &id, sizeof(grn_id), NULL); }
              ^~
    db.c:2430:7: note: remove the 'if' if its condition is always true
          WITH_NORMALIZE(hash, key, key_size, {
          ^
    db.c:46:5: note: expanded from macro 'WITH_NORMALIZE'
        if ((nstr = grn_string_open(ctx, key, key_size,\
        ^
    db.c:2429:16: note: initialize the variable 'id' to silence this warning
          grn_id id;
                   ^
                    = 0