fix crash when building songbook without songs

songbook-makeindex.py

@@ -85,10 +85,13 @@ def processSXD(filename):
     type = data[0]
     i = 1
     idx = index()
+
+    if len(data) > 1:
         while data[i].startswith('%'):
             keywords = keywordPattern.match(data[i]).groups()
             idx.keyword(keywords[0],keywords[1])
             i += 1
+
     idx.compileKeywords()
     for i in range(i,len(data),3):
         entry = processSXDEntry(data[i:i+3])