fix division by zero in target_read_string()

Under certain circumstances, a floating point exception in
target_read_string() can happen when the type has been obtained
by a call to stpy_lazy_string_elt_type(). In the latter function,
a call to check_typedef() has been forgotten. This makes
type->length = 0 in this case.

gdb/python/py-lazy-string.c

@@ -266,7 +266,7 @@ stpy_lazy_string_elt_type (lazy_string_object *lazy)
     {
     case TYPE_CODE_PTR:
     case TYPE_CODE_ARRAY:
-      return realtype->target_type ();
+      return check_typedef (realtype->target_type ());
     default:
       /* This is done to preserve existing behaviour.  PR 20769.
 	 E.g., gdb.parse_and_eval("my_int_variable").lazy_string().type.  */