do not run past the end of the disk!

[ cherry-picked de6075ff ] Conflicts: fs.c

do not run past the end of the disk!
96b7da5f · Nickolai Zeldovich · f70ef994 · 96b7da5f
--- a/fs.c
+++ b/fs.c
@@ -61,7 +61,7 @@ balloc(uint dev)
  readsb(dev, &sb);
  for(b = 0; b < sb.size; b += BPB){
    bp = bread(dev, BBLOCK(b, sb.ninodes));
-    for(bi = 0; bi < BPB; bi++){
+    for(bi = 0; bi < BPB && bi < (sb.size - b); bi++){
      m = 1 << (bi % 8);
      if((bp->data[bi/8] & m) == 0){  // Is block free?
        bp->data[bi/8] |= m;  // Mark block in use on disk.