This problem seems to exist since the x86_64 variant of memset was
improved years ago (certainly 2.11 already has the problem, as well as current
git).  There are several signed compares of the size argument, whereas
it really is unsigned.  Depending on situations e.g. a "memset(ptr, 0, -1)"
segfault (but for the wrong reasons, because jumping into nirvana) or
succeeds even.

In normal use this is harmless, as a size with signbit set indicates more than
half the address space which on x86_64 is impossible to allocate, but as the
size is used to index some jump tables this potentially could have other
unwanted side effects.