Fix out of boundary write on 1x1 terminals

If a terminal is merely 1x1 in size, setsize() will write a nul byte in
front of the allocated memory, which is an out of boundary write.

tload.c

@@ -71,6 +71,8 @@ static void setsize(int i)
 			nrows = win.ws_row;
 	}
 	scr_size = nrows * ncols;
+	if (scr_size < 2)
+		xerr(EXIT_FAILURE, _("screen too small"));
 	if (screen == NULL)
 		screen = (char *)xmalloc(scr_size);
 	else