avcodec/mpeg12: Fix error return

Also remove a tab
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg12.c

@@ -335,7 +335,7 @@ end:
     }
 
     if (i > MAX_INDEX)
-        i = AVERROR_INVALIDDATA;
+        return AVERROR_INVALIDDATA;
 
     block_last_index[index] = i;
     return 0;