`BinaryToHex` loop ignores `length` parameter, hardcodes `SHA_DIGEST_LENGTH`

[Al2Klimov]

File: lib/base/tlsutility.cpp · Function: BinaryToHex

Problem

BinaryToHex(const unsigned char* data, size_t length) accepts an arbitrary
length but the loop body iterates over the hardcoded constant
SHA_DIGEST_LENGTH (20) instead:

String BinaryToHex(const unsigned char* data, size_t length) {
    static const char hexdigits[] = "0123456789abcdef";

    String output(2 * length, 0);
    for (int i = 0; i < SHA_DIGEST_LENGTH; i++) {   // ← should be `length`
        output[2 * i]     = hexdigits[data[i] >> 4];
        output[2 * i + 1] = hexdigits[data[i] & 0xf];
    }

    return output;
}

Both current callers happen to pass exactly 20 bytes, so nothing breaks today.
But the function is declared in the public header lib/base/tlsutility.hpp and
looks like the intended generic replacement for the hand-rolled sprintf hex
loops elsewhere in the codebase. The next caller who passes a different length
(SHA-256 → 32 bytes, HMAC token → 16 bytes, …) will get silently wrong output
or out-of-bounds behaviour with no compiler warning.

[julianbrost]

for (int i = 0; i < SHA_DIGEST_LENGTH; i++) { // ← should be `length`

That comment could also have been a PR :)

I'd say that's a quick and uncontroversial change. Bonus points for some tests.

[julianbrost]

I started writing a comment here, but then realized that it would be quicker if I just fix it myself... #10841