[d16-4] [runtime] Don't zero-terminate after the string buffer. Fixes #7564.

[monojenkins]

Fixes #7564.

Backport of #7571.

/cc @rolfbjarne

[stephen-hawley]

Since I don't know the history of the file, it looks like this issue is the result of using a pattern of "check for full, then add" and failing to check.
Would it be better to make that pattern really obvious will something like:

if (FailOnBufferAtCapacity (struct_name, c, max_char, "useful error message"))
    return false;
...
static bool FailOnBufferAtCapacity(char *struct_name, int i, int max_char, char *message) {
    if (i == max_char) {
        LOGZ(message);
        struct_name[0] = 0;
        return true;
    }
    return false;
}

[monojenkins]

Build success
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[chamons]

We'll let this go out with xcode 11.3 branch.