PGS Subs: Don't read from null ptr

I saw this on the forum: http://pastebin.com/gHEjDrAz basically
```
21:36:03 T:2462055232   ERROR: ffmpeg[92BFFB40]: [pgssub] Subtitle out of video bounds. x = 65509, y = 836, video width = 1920, video height = 1080.
21:36:03 T:2462055232   ERROR: ffmpeg[92BFFB40]: [pgssub] Bitmap dimensions larger than video.
```

In that case 
```
uint8_t* s = rect.pict.data[0];
```
gets NULL and we try to read rect.w from it. 

Please someone with PGS experience (this code is there since the svn to git merge) have a look if this is the correct way to "workaround" it.

@t-nelson: You were the commiter back at that time.