screen: don't push the longer-line indicator ($) to the next row

If the last two columns of a row would be taken up by a double-width
character (and the line is longer than that), don't print it, because
it wouldn't leave any room for the $ character.

This fixes https://savannah.gnu.org/bugs/?50491.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

src/winio.c

@@ -1928,6 +1928,10 @@ char *display_string(const char *buf, size_t start_col, size_t span,
     /* Null-terminate converted. */
     converted[index] = '\0';
 
+    /* If there is more text than can be shown, make room for the $. */
+    if (*buf != '\0' && isdata && !ISSET(SOFTWRAP))
+	span--;
+
     /* Make sure converted takes up no more than span columns. */
     index = actual_x(converted, span);
     null_at(&converted, index);