prevent from panic encoding scalar types

[Alma-media]

The encoder panics while encoding values explicitly since there is no previous rune:

package main

import (
	"strings"

	"github.com/francoispqt/gojay"
)

func main() {
	var (
		buff    strings.Builder
		encoder = gojay.BorrowEncoder(&buff)
	)
	defer encoder.Release()

	encoder.Float64(42) // or AddFloat ... AddXxxx ...
}

the output will be:

panic: runtime error: index out of range [-1]

goroutine 1 [running]:
github.com/francoispqt/gojay.(*Encoder).getPreviousRune(...)
	/tmp/gopath957066451/pkg/mod/github.com/francoispqt/gojay@v1.2.13/encode.go:201
github.com/francoispqt/gojay.(*Encoder).Float64(0xc0000bb920, 0x4045000000000000)
	/tmp/gopath957066451/pkg/mod/github.com/francoispqt/gojay@v1.2.13/encode_number_float.go:124 +0x205
github.com/francoispqt/gojay.(*Encoder).Float(...)
	/tmp/gopath957066451/pkg/mod/github.com/francoispqt/gojay@v1.2.13/encode_number_float.go:61
github.com/francoispqt/gojay.(*Encoder).AddFloat64(...)
	/tmp/gopath957066451/pkg/mod/github.com/francoispqt/gojay@v1.2.13/encode_number_float.go:112
main.main()
	/tmp/sandbox969752132/prog.go:17 +0x85

fix: check the length of the buffer in order to avoid panics