[text] allow calling 'extract_iter' with invalid arguments

gallery_dl/text.py

@@ -160,10 +160,10 @@ def extract_all(txt, rules, pos=0, values=None):
 
 def extract_iter(txt, begin, end, pos=0):
     """Yield values that would be returned by repeated calls of extract()"""
-    index = txt.index
-    lbeg = len(begin)
-    lend = len(end)
     try:
+        index = txt.index
+        lbeg = len(begin)
+        lend = len(end)
         while True:
             first = index(begin, pos) + lbeg
             last = index(end, first)

test/test_text.py

@@ -300,6 +300,12 @@ class TestText(unittest.TestCase):
         self.assertEqual(
             g(txt, "[", "]", 6), ["a", "d"])
 
+        # invalid arguments
+        for value in INVALID:
+            self.assertEqual(g(value, "<"  , ">")  , [])
+            self.assertEqual(g(txt  , value, ">")  , [])
+            self.assertEqual(g(txt  , "<"  , value), [])
+
     def test_extract_from(self, f=text.extract_from):
         txt = "[c][b][a]: xyz! [d][e"