[Bug sanitizer/81986] sanitizer detects negation of large number in string.c

[zeccav at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81986

--- Comment #2 from Vittorio Zecca <zeccav at gmail dot com> ---
I do not know if this is a libgfortran or a sanitizer bug.

What I do know is that changing string.c:199 from

t = - n;

into

t = -(GFC_UINTEGER_LARGEST)n;

as it should be, the sanitizer message disappears and the whole original test
case 
pr66311.f90 runs smoothly.