[CVE-2024-34459] Fix buffer overread with xmllint --htmlout

Add a missing bounds check.

Fixes #720.

xmllint.c

@@ -543,7 +543,7 @@ xmlHTMLPrintFileContext(xmlParserInputPtr input) {
     len = strlen(buffer);
     snprintf(&buffer[len], sizeof(buffer) - len, "\n");
     cur = input->cur;
-    while ((*cur == '\n') || (*cur == '\r'))
+    while ((cur > base) && ((*cur == '\n') || (*cur == '\r')))
 	cur--;
     n = 0;
     while ((cur != base) && (n++ < 80)) {