Comment 2 for bug 1987266

Ah my previous debugging was confused, feraiseexcept is not being reached and the issue is that the algorithm depends on signed overflow at https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/ieee754/ldbl-128ibm/s_llroundl.c;h=d85154e73ac062e6c9ed403fe3cfc3889726ff72;hb=2b274fd8c9c776cf70fcdb8356e678ada522a7b0#l86 which is of course undefined behaviour. I don't completely understand the algorithm so don't really know how to fix this (or, honestly, how much I care about edge cases in ldbl-128ibm arithmetic...)