Added checks for out-of-bound seeks

- out-of-bound read results in eof
- out-of-bound write will fill missing area with zeros

The write behaviour matches expected posix behaviour, but was
under consideration for not being dropped, since littlefs does
not support holes, and support of out-of-band seeks adds complexity.

However, it turned out filling with zeros was trivial, and only
cost an extra 74 bytes of flash (0.48%).

tests/test_seek.sh

@@ -305,5 +305,43 @@ tests/test.py << TEST
     lfs_unmount(&lfs) => 0;
 TEST
 
+echo "--- Out-of-bounds seek ---"
+tests/test.py << TEST
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file[0], "hello/kitty42", LFS_O_RDWR) => 0;
+
+    size = strlen("kittycatcat");
+    lfs_file_size(&lfs, &file[0]) => $LARGESIZE*size;
+    lfs_file_seek(&lfs, &file[0],
+            ($LARGESIZE+$SMALLSIZE)*size, LFS_SEEK_SET) => 0;
+    lfs_file_read(&lfs, &file[0], buffer, size) => 0;
+
+    memcpy(buffer, "porcupineee", size);
+    lfs_file_write(&lfs, &file[0], buffer, size) => size;
+
+    lfs_file_seek(&lfs, &file[0],
+            ($LARGESIZE+$SMALLSIZE)*size, LFS_SEEK_SET) =>
+            ($LARGESIZE+$SMALLSIZE+1)*size;
+    lfs_file_read(&lfs, &file[0], buffer, size) => size;
+    memcmp(buffer, "porcupineee", size) => 0;
+
+    lfs_file_seek(&lfs, &file[0],
+            $LARGESIZE*size, LFS_SEEK_SET) =>
+            ($LARGESIZE+$SMALLSIZE+1)*size;
+    lfs_file_read(&lfs, &file[0], buffer, size) => size;
+    memcmp(buffer, "\0\0\0\0\0\0\0\0\0\0\0", size) => 0;
+
+    lfs_file_seek(&lfs, &file[0],
+            -(($LARGESIZE+$SMALLSIZE)*size), LFS_SEEK_CUR) => LFS_ERR_INVAL;
+    lfs_file_tell(&lfs, &file[0]) => ($LARGESIZE+1)*size;
+
+    lfs_file_seek(&lfs, &file[0],
+            -(($LARGESIZE+2*$SMALLSIZE)*size), LFS_SEEK_END) => LFS_ERR_INVAL;
+    lfs_file_tell(&lfs, &file[0]) => ($LARGESIZE+1)*size;
+
+    lfs_file_close(&lfs, &file[0]) => 0;
+    lfs_unmount(&lfs) => 0;
+TEST
+
 echo "--- Results ---"
 tests/stats.py