fix issue in XML parser

Author: arvidn

ChangeLog

@@ -1,5 +1,6 @@
 2.0.12
 
+	* fix issue in XML parser
 	* when building against OpenSSL, require at least version 1.1.1
 	* add setting to set no-copy-on-write flag on new files
 	* add performance counters to file pool

src/xml_parse.cpp

@@ -106,6 +106,11 @@ namespace libtorrent {
 			}
 			else if (*start == '?' && *(p - 1) == '?')
 			{
+				if (start == p - 1)
+				{
+					callback(xml_parse_error, "invalid declaration tag", {});
+					break;
+				}
 				++start;
 				callback(xml_declaration_tag, {start, std::size_t(std::min(tag_name_end - start, p - start - 1))}, {});
 				tag_end = p - 1;

test/test_xml.cpp

@@ -491,3 +491,7 @@ TORRENT_TEST(unterminated_tag_with_attribute)
 	test_parse("<foo a=\"bar", "Punexpected end of file");
 }
 
+TORRENT_TEST(invalid_declaration)
+{
+	test_parse("<?>", "Pinvalid declaration tag");
+}