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Use variable width format specifier

When using sprintf, you can use a variable width specifier, and also a "pad with zero" specifier, so that you don't need to do these things manually:

void make_hex_string_easy(unsigned int invokeid, char** xref)
{
 int len = num_hex_digits(invokeid);
 /* if odd number, add 1 - zero pad number */
 if(len % 2 != 0)
 len++;
 sprintf(*xref, "%0*X", len, invokeid);
}

Here, after the % character, the 0 tells sprintf to pad the hex value with zeros. For example:

printf("%08x", 0x1234); // Will print "00001234" due to the %08x specifier

After the 0 there is a * character. This tells sprintf that the width will come from a variable instead of being hardcoded. For example:

printf("%0*x", 6, 0x1234); // Will print "001234" with a width of 6

So putting it all together, "%0*X" prints a zero padded hex string whose width comes from the next argument. In the modified code above, that argument is len.