Switch OBJ_undef uses to NID_undef.

They both happen to be zero, but OBJ_undef is a type error; OBJ_foo expands to
a comma-separated list of integers.

Change-Id: Ia5907dd3bc83240b7cc98af6456115d2efb48687
Reviewed-on: https://boringssl-review.googlesource.com/2842
Reviewed-by: Adam Langley <agl@google.com>
2 files changed