[Bug sanitizer/60613] Invalid signed subtraction ubsan diagnostics

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60613

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 32423
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32423&action=edit
gcc49-pr60613.patch

Untested fix.