lyrics: Add some missing None-checks to the Tekstowo source

The previous code had the potential to crash if (when?) Tekstowo changes
their website structure sufficiently.

The new code is rather ugly due to the explicit checks after each and
every function call. Unfortunately, the alternative would be to catch a
bunch of very generic Exceptions (AttributeError, ...), since there's no
such thing as a `BeautifulSoupNotFoundError`.

beetsplug/lyrics.py

@@ -461,20 +461,27 @@ class Tekstowo(Backend):
         if not soup:
             return None
 
-        song_rows = soup.find("div", class_="content"). \
-            find("div", class_="card"). \
-            find_all("div", class_="box-przeboje")
+        content_div = soup.find("div", class_="content")
+        if not content_div:
+            return None
 
+        card_div = content_div.find("div", class_="card")
+        if not card_div:
+            return None
+
+        song_rows = card_div.find_all("div", class_="box-przeboje")
         if not song_rows:
             return None
 
         song_row = song_rows[0]
-
         if not song_row:
             return None
 
-        href = song_row.find('a').get('href')
-        return self.BASE_URL + href
+        link = song_row.find('a')
+        if not link:
+            return None
+
+        return self.BASE_URL + link.get('href')
 
     def extract_lyrics(self, html):
         html = _scrape_strip_cruft(html)
@@ -484,10 +491,11 @@ class Tekstowo(Backend):
         if not soup:
             return None
 
-        c = soup.find("div", class_="song-text")
-        if c:
-            return c.get_text()
-        return None
+        lyrics_div = soup.find("div", class_="song-text")
+        if not lyrics_div:
+            return None
+
+        return lyrics_div.get_text()
 
 
 def remove_credits(text):