write_redis: fix build warning

src/write_redis.c: In function ‘wr_config’:
src/write_redis.c:222:53: warning: ‘%s’ directive output may be truncated writing up to 127 bytes into a region of size 116 [-Wformat-truncation=]
     snprintf(cb_name, sizeof(cb_name), "write_redis/%s", node->name);
                                                     ^~
src/write_redis.c:222:5: note: ‘snprintf’ output between 13 and 140 bytes into a destination of size 128
     snprintf(cb_name, sizeof(cb_name), "write_redis/%s", node->name);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~