BF: place a mutex around apparently thread-unsafe parser.feed invocation

That leads to fix up of anchors analysis and probably other issues
such as floating number of found urls etc

linkcheck/parser/__init__.py

@@ -17,11 +17,16 @@
 """
 Main functions for link parsing
 """
+import threading
+
 from .. import log, LOG_CHECK, strformat, url as urlutil
 from ..htmlutil import linkparse
 from ..HtmlParser import htmlsax
 from ..bookmarks import firefox
 
+# Is needed within find_links around non-threadsafe call
+parse_mutex = threading.Lock()
+
 
 def parse_url(url_data):
     """Parse a URL."""
@@ -125,7 +130,9 @@ def find_links (url_data, callback, tags):
     handler.parser = parser
     # parse
     try:
-        parser.feed(url_data.get_content())
+        content = url_data.get_content()
+        with parse_mutex:
+            parser.feed(content)
         parser.flush()
     except linkparse.StopParse as msg:
         log.debug(LOG_CHECK, "Stopped parsing: %s", msg)