message: file-backed-mmap: additional correctness checks
diff: #include "wrappers/mmapwrapper.h"
  #include "wrappers/stlallocator.h"
  +#include "internal.h"
  +
  #ifndef HL_MMAP_PROTECTION_MASK
  #error "define HL_MMAP_PROTECTION_MASK before including heaplayers.h"
  #endif
  @@ -87,10 +89,17 @@ public:
  sprintf(buf, "/tmp/alloc-mesh-%d/XXXXXX", getpid());
  int fd = mkstemp(buf);
  - if (fd < 0)
  + if (fd < 0) {
  + debug("mkstemp: %s\n", strerror(errno));
  abort();
  + }
  +
  + int err = ftruncate(fd, sz);
  + if (err != 0) {
  + debug("ftruncate: %s\n", strerror(errno));
  + abort();
  + }
  - ftruncate(fd, sz);
  void *ptr = mmap(nullptr, sz, HL_MMAP_PROTECTION_MASK, MAP_SHARED, fd, 0);
  if (ptr == MAP_FAILED)
  