The bound on ures <= 0x80[...] should be either ures < 0x80[...] or ures <= 0x7F[...]. Otherwise, parsing negative `0x8000000000000000` will run code to negate the signed integer INT64_MIN (-0x80[...]) to 0x80[...], which doesn't fit in an integer (INT*_MAX is 0x80[...]).

    $ clang parseint.c -fsanitize=undefined -O0 -g -o parseint
    $ ./parseint -9223372036854775808
    parseint.c:38:23: runtime error: negation of -9223372036854775808 cannot be represented in type 'long long'; cast to an unsigned type to negate this value to itself
    result: -9223372036854775808

edit: this is just to show that getting undefined behavior right is hard!