Confirm the symbol table retrieved is not None

It may be None in the case of a list comprehension being declared
in a class scope (not in a function of a class). This, however,
is not valid python syntax.

Author: dhood

mypy/semanal.py

@@ -4550,7 +4550,10 @@ def current_symbol_table(self, escape_comprehensions: bool = False) -> SymbolTab
                             # The caller of the comprehension is in the global space.
                             names = self.globals
                         else:
-                            names = cast(SymbolTable, self.locals[-1 - i])
+                            names_candidate = self.locals[-1 - i]
+                            assert names_candidate is not None, \
+                                "Escaping comprehension from invalid scope"
+                            names = names_candidate
                         break
                 else:
                     assert False, "Should have at least one non-comprehension scope"