KF8 Input: Do not link to font files that we failed to properly extract

src/calibre/ebooks/mobi/reader/markup.py

@@ -154,6 +154,8 @@ def update_flow_links(mobi8_reader, resource_map, log):
                     'valid font in %s' % (num, tag))
                 else:
                     replacement = '"%s"'%('../'+ href)
+                    if href.endswith('.dat'):
+                        replacement = 'unable-to-extract.ttf'
                     tag = font_index_pattern.sub(replacement, tag, 1)
 
             # process links to other css pieces