Fix lynx segmentation fault

[Alexey Dobriyan <adobriyan_(_at_)_gmail_(_dot_)_com>]

Steps to reproduce:

	echo '<TAB INDENT=1 TO>' >tab_indent_to.html
	lynx tab_indent_to.html

lynx coredumps.

--- src/HTML.c
+++ src/HTML.c
@@ -2091,7 +2091,8 @@ PRIVATE int HTML_start_element ARGS6(
 	    int enval = 2;
 
 	    column = HText_getCurrentColumn(me->text);
-	    if (present[HTML_TAB_TO]) {
+	    if (present[HTML_TAB_TO] &&
+		value[HTML_TAB_TO] && *value[HTML_TAB_TO]) {
 		/*
 		 *  TO has priority over INDENT if both are present. - FM
 		 */