Remove -2 return value from X509*_get_*_by_NID.

X509*_get_*_by_NID return -1 if the extension was not found, but -2 if
the NID was invalid. Looking through callers, many check index != -1,
rather than index < 0. That means, in theory, they'll do the wrong thing
in some cases.

Realistically, this case is impossible: most callers pass in a constant.
Even in those that don't, NIDs are a local enum, not standard constants.
That means hitting this path is almost certainly a programmer error. No
need to complicate the calling convention for it.

Update-Note: The return value convention of some functions was
simplified. This is not expected to affect any callers.

Change-Id: If2f5a45c37caccdbfcc3296ff2db6db1183e3a95
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48368
Reviewed-by: Adam Langley <agl@google.com>

crypto/x509/x509_att.c

diff --git a/crypto/x509/x509_att.c b/crypto/x509/x509_att.c
index d3e78c3..e2a5121 100644
--- a/crypto/x509/x509_att.c
+++ b/crypto/x509/x509_att.c
@@ -76,11 +76,7 @@
 {
     const ASN1_OBJECT *obj = OBJ_nid2obj(nid);
     if (obj == NULL) {
-        /* TODO(davidben): Return -1 here. Callers often check the result
-         * against -1 without handling a theoretical -2 output. Reporting a
-         * different result should not be necessary because invalid NIDs are
-         * almost always a programmer error.  */
-        return -2;
+        return -1;
     }
     return X509at_get_attr_by_OBJ(x, obj, lastpos);
 }

crypto/x509/x509_v3.c

diff --git a/crypto/x509/x509_v3.c b/crypto/x509/x509_v3.c
index d2a51c0..00d6399 100644
--- a/crypto/x509/x509_v3.c
+++ b/crypto/x509/x509_v3.c
@@ -74,11 +74,7 @@
 {
     const ASN1_OBJECT *obj = OBJ_nid2obj(nid);
     if (obj == NULL) {
-        /* TODO(davidben): Return -1 here. Callers often check the result
-         * against -1 without handling a theoretical -2 output. Reporting a
-         * different result should not be necessary because invalid NIDs are
-         * almost always a programmer error.  */
-        return -2;
+        return -1;
     }
     return X509v3_get_ext_by_OBJ(x, obj, lastpos);
 }