don't garble partial multi-byte character after control sequence

When use [lf](https://github.com/gokcehan/lf) to list files, emacs-libvterm may
read partial multi-byte character, for example:

  $ echo -n '招聘' | hexdump -C
  00000000  e6 8b 9b e8 81 98

  ; get "招", control sequence and partial character
  (vterm--filter process "\xE6\x8B\x9B\e[14;111H\xE8")

  ; now full "聘"
  (vterm--filter process "\x81\x98")

This will send "\xE8" to libvterm which is not a full character.

1 files changed, 1 insertions, 1 deletions

diff --git a/vterm.el b/vterm.el
index 04c9058..fff90f2 100644
--- a/vterm.el
+++ b/vterm.el
@@ -1522,7 +1522,7 @@ Then triggers a redraw from the module."
                                                       (- count 1 partial)))
                                   'eight-bit))
                     (cl-incf partial))
-                  (when (> count partial 0)
+                  (when (> (1+ count) partial 0)
                     (setq vterm--undecoded-bytes
                           (substring decoded-substring (- partial)))
                     (setq decoded-substring