grow buffer for string

[shogo82148]

No description provided.

[shogo82148]

:(

goos: darwin
goarch: arm64
pkg: github.com/shogo82148/go-sfv
             │  .old.txt   │           .new.txt            │
             │   sec/op    │   sec/op     vs base          │
EncodeItem-8   699.9n ± 1%   698.9n ± 1%  ~ (p=0.565 n=10)

             │   .old.txt   │             .new.txt             │
             │     B/op     │     B/op      vs base            │
EncodeItem-8   2.500Ki ± 0%   2.500Ki ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

             │  .old.txt  │            .new.txt            │
             │ allocs/op  │ allocs/op   vs base            │
EncodeItem-8   3.000 ± 0%   3.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal