Handle completely undecoded input in term (Bug#29918)

* lisp/term.el (term-emulate-terminal): Avoid errors if the whole
decoded string is eight-bit characters.  Don't attempt to save the
string for next iteration in that case.
* test/lisp/term-tests.el (term-decode-partial)
(term-undecodable-input): New tests.

1 files changed, 5 insertions, 4 deletions

diff --git a/lisp/term.el b/lisp/term.el
index cbef68dc0ac..9785ce30249 100644
--- a/lisp/term.el
+++ b/lisp/term.el
@@ -2900,11 +2900,12 @@ See `term-prompt-regexp'."
                           ;; next time.
                           (when (= funny str-length)
                             (let ((partial 0))
-                              (while (eq (char-charset (aref decoded-substring
-                                                             (- count 1 partial)))
-                                         'eight-bit)
+                              (while (and (< partial count)
+                                          (eq (char-charset (aref decoded-substring
+                                                                  (- count 1 partial)))
+                                              'eight-bit))
                                 (cl-incf partial))
-                              (when (> partial 0)
+                              (when (> count partial 0)
                                 (setq term-terminal-undecoded-bytes
                                       (substring decoded-substring (- partial)))
                                 (setq decoded-substring