posix_memalign and malloc canaries don't mix

[Jeremy Evans]

posix_memalign doesn't seem to work with malloc canaries
in some allocations.  This appears to be what caused the
issue malloc canary failure in ruby when the page size
was accidentally set to high.  Example:

#include <stdlib.h>

char *malloc_options = "C";

int main(int argc, char** argv){
  void *v;
  posix_memalign(&v, 8192, 8192-40);
  free(v);
  exit(0);
}