vinyl: fix warnings if compiled with NDEBUG

src/box/vinyl.c: In function ‘vy_range_write_run’:
src/box/vinyl.c:2365:1: error: label ‘create_failed’ defined but not used [-Werror=unused-label]
 create_failed:
 ^
src/box/vinyl.c: In function ‘vy_task_compact_execute’:
src/box/vinyl.c:3190:31: error: suggest braces around empty body in an ‘if’ statement [-Werror=empty-body]
           rc = -1; goto out;});
                               ^
src/box/vinyl.c:3208:1: error: label ‘unlink_error’ defined but not used [-Werror=unused-label]
 unlink_error:
 ^
src/box/vinyl.c: In function ‘vy_mem_iterator_close’:
src/box/vinyl.c:6829:26: error: unused variable ‘itr’ [-Werror=unused-variable]
  struct vy_mem_iterator *itr = (struct vy_mem_iterator *) vitr;
                          ^
src/box/vinyl.c: In function ‘vy_txw_iterator_close’:
src/box/vinyl.c:7087:26: error: unused variable ‘itr’ [-Werror=unused-variable]
  struct vy_txw_iterator *itr = (struct vy_txw_iterator *) vitr;
                          ^