Fix functions like `subtract_whole` requiring `calloc` allocated memory since this is inefficient.

[avighnac]

subtract_whole.asm:82:93

.after_if_3:
  push   rdi
  push   rcx
  mov    rdi, rdx
  call   strlen_asm wrt ..plt
  pop    rcx
  pop    rdi
  test   cl, cl
  jz     .ret
  mov    byte [rdx+rax+1], 0
  lea    r8, [rax-1]
  xor    ecx, ecx

This code doesn't really do anything (the string is already null terminated, otherwise the call to strlen_asm wouldn't work), and doesn't need to call strlen_asm at all. Fix functions like this so they null terminate efficiently and properly.