Unfortunately the usage of HI results in about 25000 compiler warnings on GCC < 5 (which defaults to C90):
warning: this decimal constant is unsigned only in ISO C90 [enabled by default]
as well as an assertion failure "slnum >= 0" in warnless.c:curlx_sltoui.
Maybe LONG_MAX or INT_MAX from limits.h could be used or just that number - 1?