Check for errors in a2d_ASN1_OBJECT()

Check for error return in BN_div_word().

(Imported from upstream's d871284aca5524c85a6460119ac1b1e38f7e19c6.)

This function is only called from crypto/obj to convert strings like
"1.2.3.4.5" to OIDs. We may wish to see about rewriting it just so it's
out of the way.

Change-Id: Ia8379d2dd30606f6a81ce24dee8852312cb7f127
Reviewed-on: https://boringssl-review.googlesource.com/10365
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/asn1/a_object.c

diff --git a/crypto/asn1/a_object.c b/crypto/asn1/a_object.c
index 561eb14..fef9b79 100644
--- a/crypto/asn1/a_object.c
+++ b/crypto/asn1/a_object.c
@@ -172,8 +172,12 @@
                 if (!tmp)
                     goto err;
             }
-            while (blsize--)
-                tmp[i++] = (unsigned char)BN_div_word(bl, 0x80L);
+            while (blsize--) {
+                BN_ULONG t = BN_div_word(bl, 0x80L);
+                if (t == (BN_ULONG)-1)
+                    goto err;
+                tmp[i++] = (unsigned char)t;
+            }
         } else {
 
             for (;;) {