don't dereference this if it doesn't exist in dvdnav_internal.h

[Erik Hovland <erik@…>]

In dvdnav_internal.h, printerr is defined as:

#define printerr(str) strncpy(this->err_str, str, MAX_ERR_LEN);

Unfortunately, all over the code functions call:

  if(!this) {
    printerr("Passed a NULL pointer.");
    return DVDNAV_STATUS_ERR;
  }

Which means that instead of printing an error it will try to dereference a null pointer and segfault.

[Erik Hovland <erik@…>]

check this before dereferencing it

[danielk]

(In [16438]) Fixes #4844. Fixes some broken printerr() macros in dvdnav that produced segfaults on some DVD error conditions.