Remove vsnprintf() and strtoumax() validation.

Remove code that validates malloc_vsnprintf() and malloc_strtoumax()
against their namesakes.  The validation code has adequately served its
usefulness at this point, and it isn't worth dealing with the different
formatting for %p with glibc versus other implementations for NULL
pointers ("(nil)" vs. "0x0").

Reported by Mike Hommey.