XML: bad fix for a crash that only happened in XHTML.

As an error is output, I suppose that this behaviour is OK.

src/xml.cpp

@@ -477,6 +477,7 @@ XMLStream &XMLStream::operator<<(xml::EndTag const &etag)
 						   + "' when other tags were pending. Last pending tag is `"
 						   + to_utf8(pending_tags_.back()->writeTag())
 						   + "'. Tag discarded.");
+				if (!pending_tags_.empty())
 					pending_tags_.erase(dit);
 				return *this;
 			}