crashed with SIGSEGV in notify.c's notifyCallback()

[charles]

Downstream ticket reported by RuiDC

​https://bugs.launchpad.net/ubuntu/+source/transmission/+bug/334104

I'm not exactly sure *how* it happened -- I think that part may be fixed in newer releases -- but looking at the stack trace, it's clearly a crash being caused by dereferencing a null pointer:

#0 notifyCallback (n=<value optimized out>, action=<value optimized out>, gdata=<value optimized out>) at notify.c:54 tor = (tr_torrent *) 0x0 info = (const tr_info *) 0x0

and, a few lines later, we have this:

char * path = g_build_filename( dir, info->files[0].name, NULL );

We can prevent the crash by adding some NULL safeguards.

[charles]

fixed in r8670 for 1.72