movdec: Fix bad computed size for atoms with size 0 in MOV files

The computed size doesn't contain the header size because it's already
skipped by incrementing total_size, but then it's skipped again in the
last line. The atom comes out 8 bytes short and the function
mov_read_chan() aborts the whole parsing process. I think the computed
size should be atom.size - total_size + 8.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -323,7 +323,7 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
             total_size += 8;
         }
         if (a.size == 0) {
-            a.size = atom.size - total_size;
+            a.size = atom.size - total_size + 8;
             if (a.size <= 8)
                 break;
         }