Fix handling of sbrk(0)

We insert dummy empty elements into crange and leak vmas in radix.

Fix handling of sbrk(0)
53de149d · David Benjamin · f014b06a · 53de149d
--- a/kernel/vm.cc
+++ b/kernel/vm.cc
@@ -685,12 +685,12 @@ vmap::sbrk(ssize_t n, uptr *addr)
  auto curbrk = brk_;
  *addr = curbrk;

-  if(n < 0 && 0 - n <= curbrk){
+  if(n <= 0 && 0 - n <= curbrk){
    brk_ += n;
    return 0;
  }

-  if(n < 0 || n > USERTOP || curbrk + n > USERTOP)
+  if(n <= 0 || n > USERTOP || curbrk + n > USERTOP)
    return -1;

  // look one page ahead, to check if the newly allocated region would