perf: improve validate mesg sec/op delta by -72%

[muktihari]

• remove the need to allocate new slices improve the performance by an order of magnitude.

goos: darwin
goarch: amd64
pkg: github.com/muktihari/fit/encoder
cpu: Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
           │   old.txt    │               new.txt               │
           │    sec/op    │   sec/op     vs base                │
Validate-4   147.70n ± 3%   40.28n ± 0%  -72.73% (p=0.000 n=10)

           │  old.txt   │              new.txt               │
           │    B/op    │   B/op     vs base                 │
Validate-4   64.00 ± 0%   0.00 ± 0%  -100.00% (p=0.000 n=10)

           │  old.txt   │               new.txt               │
           │ allocs/op  │ allocs/op   vs base                 │
Validate-4   1.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)