AZW3 Input: Do not fail to process files with invalid internal flow references.

Fixes #1740140 [calibre, version 3.14.0 ERRO: Could not open e-book: Falha ao ler o livro, C:\Users\Admin\Google Drive\Work\Calibre\Livros\Allan Kardec\O Evangelho Segundo o Espiritismo (3146)\O Evangelho Segundo o Espiritis - Allan Kardec.azw3 clique em "Mostrar detalhes" para mais informações.  Traceback (most recent call last):   File "site-packages\calibre\utils\ipc\simple_worker.py", line 284, in main   File "site-packages\calibre\ebooks\oeb\iterator\book.py", line 64, in extract_book   File "site-packages\calibre\customize\conversion.py", line 245, in __call__   File "site-packages\calibre\ebooks\conversion\plugins\mobi_input.py", line 44, in convert   File "site-packages\calibre\ebooks\mobi\reader\mobi8.py", line 114, in __call__   File "site-packages\calibre\ebooks\mobi\reader\mobi8.py", line 465, in expand_text   File "site-packages\calibre\ebooks\mobi\reader\markup.py", line 356, in expand_mobi8_markup   File "site-packages\calibre\ebooks\mobi\reader\markup.py", line 204, in update_flow_links IndexError: cannot fit 'long' into an index-sized integer](https://bugs.launchpad.net/calibre/+bug/1740140)

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

@@ -200,14 +200,19 @@ def update_flow_links(mobi8_reader, resource_map, log):
             tag = srcpieces[j]
             if tag.startswith('<'):
                 for m in re.finditer(flow_pattern, tag):
-                    num = int(m.group(1), 32)
-                    fi = mr.flowinfo[num]
-                    if fi.format == 'inline':
-                        flowtext = mr.flows[num]
-                        tag = flowtext
+                    try:
+                        num = int(m.group(1), 32)
+                        fi = mr.flowinfo[num]
+                    except IndexError:
+                        log.warn('Ignoring invalid flow reference in tag', tag)
+                        tag = ''
                     else:
-                        replacement = '"../' + fi.dir + '/' + fi.fname + '"'
-                        tag = flow_pattern.sub(replacement, tag, 1)
+                        if fi.format == 'inline':
+                            flowtext = mr.flows[num]
+                            tag = flowtext
+                        else:
+                            replacement = '"../' + fi.dir + '/' + fi.fname + '"'
+                            tag = flow_pattern.sub(replacement, tag, 1)
                 srcpieces[j] = tag
         flow = "".join(srcpieces)
 
@@ -390,4 +395,3 @@ def expand_mobi8_markup(mobi8_reader, resource_map, log):
                 f.write(flow.encode('utf-8'))
 
     return spine
-