patch 8.2.0060: message test only runs with one encoding

Problem:    Message test only runs with one encoding. (Dominique Pelle)
Solution:   Run the test with "utf-8" and "latin1".  Fix underflow. (related
            to #5410)

src/message.c

@@ -307,7 +307,8 @@ trunc_string(
     }
     else
     {
-	for (i = (int)STRLEN(s); len + (n = ptr2cells(s + i - 1)) <= room; --i)
+	for (i = (int)STRLEN(s);
+		   i - 1 >= 0 && len + (n = ptr2cells(s + i - 1)) <= room; --i)
 	    len += n;
     }