Handling a specific runtime error in Go loop

I'd like to catch panic: runtime error: index out of range in the following loop (inside function without returning) and concat X for each panic: runtime error: index out of range to the result:

func transform(inputString string, inputLength int) string {
    var result = ""
    for index := 0; index < inputLength; index++ {
        if string(inputString[index]) == " " {
            result = result + "%20"
        } else {
            result = result + string(inputString[index])
        }
    }
    return result
}

for example for inputString = Mr Smith and inputLength = 10 the result is Mr%20SmithXX. It has two X because 10 - 8 = 2.

I know I can catch the panic in returning from transform() but I'd like to handle it inside the loop without returning the function.

inputString = Mr Smith and inputLength = 10 the result is Mr%20SmithXX. It has two X because 10 - 8 = 2.

package main

import "fmt"

func transform(s string, tLen int) string {
    t := make([]byte, 0, 2*tLen)
    for _, b := range []byte(s) {
        if b == ' ' {
            t = append(t, "%20"...)
        } else {
            t = append(t, b)
        }
    }
    for x := tLen - len(s); x > 0; x-- {
        t = append(t, 'X')
    }
    return string(t)
}

func main() {
    s := "Mr Smith"
    tLen := 10
    fmt.Printf("%q %d\n", s, tLen)
    t := transform(s, tLen)
    fmt.Printf("%q\n", t)
}

https://go.dev/play/p/wg7MIO8yUzF

"Mr Smith" 10
"Mr%20SmithXX"