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>
diff --git a/ssl/s3_lib.c b/ssl/s3_lib.c
index 69a78e7..9782296 100644
--- a/ssl/s3_lib.c
+++ b/ssl/s3_lib.c
@@ -855,7 +855,7 @@
         int nid;
         for (i = 0; i < clistlen; i++) {
           nid = tls1_ec_curve_id2nid(clist[i]);
-          if (nid != OBJ_undef) {
+          if (nid != NID_undef) {
             cptr[i] = nid;
           } else {
             cptr[i] = TLSEXT_nid_unknown | clist[i];
diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 799f240..d2660d0 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -393,7 +393,7 @@
 
 int tls1_ec_curve_id2nid(uint16_t curve_id) {
   if (curve_id < 1 || curve_id > sizeof(nid_list) / sizeof(nid_list[0])) {
-    return OBJ_undef;
+    return NID_undef;
   }
   return nid_list[curve_id - 1];
 }