Fix simple formatting optimization

Fix simple formatting optimization

name                        old2 ns/iter  new2 ns/iter  diff ns/iter   diff %  speedup
fmt::write_str_macro1       12,295        12,308                  13    0.11%   x 1.00
fmt::write_str_macro2       24,079        21,451              -2,628  -10.91%   x 1.12
fmt::write_str_macro_debug  238,363       230,807             -7,556   -3.17%   x 1.03
fmt::write_str_ref          6,203         6,064                 -139   -2.24%   x 1.02
fmt::write_str_value        6,225         6,075                 -150   -2.41%   x 1.02
fmt::write_vec_macro1       17,144        17,121                 -23   -0.13%   x 1.00
fmt::write_vec_macro2       29,845        26,703              -3,142  -10.53%   x 1.12
fmt::write_vec_macro_debug  248,840       242,117             -6,723   -2.70%   x 1.03
fmt::write_vec_ref          5,954         6,438                  484    8.13%   x 0.92
fmt::write_vec_value        5,959         6,439                  480    8.06%   x 0.93