lavf: ffio_limit(), treat avio_size()==0 as error.

This works around issues arising from inputs that claim to have a
filesize of 0.

Reported-by: buzz_
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit e6362f3a3c)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -271,7 +271,7 @@ int ffio_limit(AVIOContext *s, int size)
         if(remaining < size){
             int64_t newsize= avio_size(s);
             if(!s->maxsize || s->maxsize<newsize)
-                s->maxsize= newsize;
+                s->maxsize= newsize - !newsize;
             remaining= s->maxsize - avio_tell(s);
             remaining= FFMAX(remaining, 0);
         }