fix compiler crash on nested generator expressions

--HG--
extra : transplant_source : l%01%83uy%F4Z%F6%AD%FBj%9E%1B%E8%C1O%0C%25R%1B

Cython/Compiler/Symtab.py

@@ -1571,7 +1571,7 @@ class GeneratorExpressionScope(Scope):
                 type = outer_entry.type # may still be 'unspecified_type' !
         # the parent scope needs to generate code for the variable, but
         # this scope must hold its name exclusively
-        cname = '%s%s' % (self.genexp_prefix, self.parent_scope.mangle(Naming.var_prefix, name))
+        cname = '%s%s' % (self.genexp_prefix, self.parent_scope.mangle(Naming.var_prefix, name or self.next_id()))
         entry = self.declare(name, cname, type, pos, visibility)
         entry.is_variable = 1
         self.var_entries.append(entry)