upstream: Fix two separate one-byte out-of-cound reads

upstream: Fix two separate one-byte out-of-cound reads

1) if a server sent an empty reply to a SSH2_FXP_REALPATH request
2) if a batch command used the full 2048 byte buffer but ended in a
  literal backslash character

Both reported by Zhenpeng (Leo) Lin from depthfirst

ok markus@

OpenBSD-Commit-ID: d1ccc1f5a6eb109065ce8a552fea8e502381ce59