[GSUB] Parse old-syntax MultipleSubst that has no index attribute

Apparently, earlier versions of TTX had produced XML whose
MultipleSubst.Sequence element came without an index attribute.
Resolves https://github.com/behdad/fonttools/issues/385

Lib/fontTools/ttLib/tables/otTables.py

@@ -254,7 +254,8 @@ class MultipleSubst(FormatSwitchingBaseTable):
 					self.old_coverage_.append(element_attrs["value"])
 			return
 		if name == "Sequence":
-			glyph = self.old_coverage_[int(attrs["index"])]
+			index = int(attrs.get("index", len(mapping)))
+			glyph = self.old_coverage_[index]
 			glyph_mapping = mapping[glyph] = []
 			for element in content:
 				if not isinstance(element, tuple):

Lib/fontTools/ttLib/tables/otTables_test.py

@@ -165,6 +165,27 @@ class MultipleSubstTest(unittest.TestCase):
         self.assertEqual(table.mapping,
                          {'c_t': ['c', 't'], 'f_f_i': ['f', 'f', 'i']})
 
+    def test_fromXML_oldFormat_bug385(self):
+        # https://github.com/behdad/fonttools/issues/385
+        table = otTables.MultipleSubst()
+        table.Format = 1
+        for name, attrs, content in parseXML(
+                '<Coverage Format="1">'
+                '  <Glyph value="o"/>'
+                '  <Glyph value="l"/>'
+                '</Coverage>'
+                '<Sequence>'
+                '  <Substitute value="o"/>'
+                '  <Substitute value="l"/>'
+                '  <Substitute value="o"/>'
+                '</Sequence>'
+                '<Sequence>'
+                '  <Substitute value="o"/>'
+                '</Sequence>'):
+            table.fromXML(name, attrs, content, self.font)
+        self.assertEqual(table.mapping,
+                         {'o': ['o', 'l', 'o'], 'l': ['o']})
+
 
 class LigatureSubstTest(unittest.TestCase):
     def setUp(self):