security/nss: unbreak build on armv6

pqg.c:339:16: error: comparison of constant 18446744073709551615 with expression of type 'unsigned long' is always true [-Werror,-Wtautological-constant-out-of-range-compare]

if (addend < MP_DIGIT_MAX) {
    ~~~~~~ ^ ~~~~~~~~~~~~

1 error generated.

PR: 209482
Submitted by: mikael.urankar@gmail.com (based on)