dbghelp: Fix stabs_pts_read_type_def() for when typename is NULL

[Andrew Talbot]

Eric Pouech wrote:

> looks like a bit strange to me that you get a null typename here
> can you send me the DLL/.so file on which you get the seg fault
> A+
> 

No known segfaults; I'm just doing static analysis. But
stabs_pts_read_type_def() is called several times within stabs.c passing
NULL as a typename argument: e.g., line 494:

    PTS_ABORTIF(ptd, stabs_pts_read_type_def(ptd, NULL, &ref) == -1);

And, with case 'u' (i.e., *ptd->ptr == 'u') and udt != NULL, it would be
passed to strlen(). I didn't establish whether this will ever occur with
typename == NULL, I just sought to make the function more bullet proof.

-- 
Andy.