Fix 'Dead nested assignment's from scan-build-10

Fix 'Dead nested assignment's from scan-build-10

The 3 warnings reported are:

os.c:872:7: warning: Although the value stored to 'ptr' is used in the enclosing expression, the value is never actually read from 'ptr'
        if ((ptr = strtok_r(command, " \t", &last)) == NULL) {
             ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

--

rpz.c:1117:10: warning: Although the value stored to 'zbits' is used in the enclosing expression, the value is never actually read from 'zbits'
        return (zbits &= x);
                ^        ~
1 warning generated.

--

openssleddsa_link.c:532:10: warning: Although the value stored to 'err' is used in the enclosing expression, the value is never actually read from 'err'
        while ((err = ERR_get_error()) != 0) {
                ^     ~~~~~~~~~~~~~~~
1 warning generated.