purple_markup_html_to_xhtml in util.c segfaults given invalid html

[jeffr]

purple_markup_html_to_xhtml, which appears to only be called when sending an XMPP IM or chat message, can crash given strange HTML. For example, the string "<a>" will cause a segfault.

test case: purple_markup_html_to_xhtml("<a>", g_string_new(""), g_string_new(""));

[jeffr]

Sorry, here is a more proper test case:

char* xhtml; char* plaintext; xhtml = g_strdup(""); plaintext = g_strdup(""); purple_markup_html_to_xhtml("<a>", &xhtml, &plaintext);

[QuLogic]

To be more specific for this example, it expects an "href=" for every "<a>". This should only crash if the href is missing in the first <a>, since otherwise, the GString it's accessing is no longer NULL. The GString is free'd when a new href is found, so it should still be a valid pointer, though it will result in totally incorrect data.

At least, that's how I see it at the moment.

[lschiere@…]

(In ​aff089bc73ecc6fe8ebbeac670db8be13511fcf4) a patch from stu for html handling.

fixes #3436