Panics when issuing keys d f n on an empty buffer

[stephenlacy]

When open on an empty buffer, hitting the keys dfn causes a panic:

thread 'main' panicked at 'Attempt to index past end of RopeSlice: char index 1, RopeSlice char length 0', /Users/runner/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/ropey-1.2.0/src/slice.rs:917:9
stack backtrace:
   0:        0x108655354 - __mh_execute_header
   1:        0x10867327e - __mh_execute_header
   2:        0x108651166 - __mh_execute_header
   3:        0x1086574f9 - __mh_execute_header
   4:        0x108656fe0 - __mh_execute_header
   5:        0x10820bb9a - __mh_execute_header
   6:        0x108657b88 - __mh_execute_header
   7:        0x1086576a5 - __mh_execute_header
   8:        0x1086557b8 - __mh_execute_header
   9:        0x10865760a - __mh_execute_header
  10:        0x108695ebb - __mh_execute_header
  11:        0x10841a6d5 - __mh_execute_header
  12:        0x1081af951 - __mh_execute_header
  13:        0x108229285 - __mh_execute_header
  14:        0x108247b57 - __mh_execute_header
  15:        0x108207d6d - __mh_execute_header
  16:        0x108243bac - __mh_execute_header
  17:        0x1081bfe8f - __mh_execute_header
  18:        0x10820e34a - __mh_execute_header
  19:        0x10825d27a - __mh_execute_header
  20:        0x10825772c - __mh_execute_header
  21:        0x108657ed4 - __mh_execute_header
  22:        0x10820e729 - __mh_execute_header

Version: 0.0.6 (prebuilt)
OS: macos