Fix infloop in shell.el

* test/lisp/shell-tests.el (shell-tests-completion-before-semi):
Amend the shell.el tests to catch errors such as bug#39057.

* lisp/shell.el (shell--parse-pcomplete-arguments): Skip the
semi-colon as well.  This avoids inflooping when a semi-colon is
typed by the user.  (Bug#39057)

Copyright-paperwork-exempt: yes

lisp/shell.el

@@ -428,7 +428,7 @@ Thus, this does not include the shell's current directory.")
     (save-excursion
       (goto-char begin)
       (while (< (point) end)
-	(skip-chars-forward " \t\n")
+	(skip-chars-forward " \t\n;")
 	(push (point) begins)
         (let ((arg ()))
           (while (looking-at

test/lisp/shell-tests.el

@@ -34,8 +34,7 @@
   (with-temp-buffer
     (shell-mode)
     (insert "cd ba;")
-    (forward-char -1)
     (should (equal (shell--parse-pcomplete-arguments)
-                   '(("cd" "ba") 1 4)))))
+                   '(("cd" "ba" "") 1 4)))))
 
 ;;; shell-tests.el ends here