Ignore unused value

Right now, compiling with the stock gcc on debian, cmake is compiling
with -Wall which gives an error because -Wunused-value.

The gcc version is gcc (Debian 4.7.2-5) 4.7.2.

Change-Id: Iafd4cc14a22fe788d4c7bdb05202fd856f0c6395
Reviewed-on: https://boringssl-review.googlesource.com/21144
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/x509/x_name.c

diff --git a/crypto/x509/x_name.c b/crypto/x509/x_name.c
index 0980463..f132e6b 100644
--- a/crypto/x509/x_name.c
+++ b/crypto/x509/x_name.c
@@ -244,7 +244,7 @@
             entry->set = i;
             if (!sk_X509_NAME_ENTRY_push(nm.x->entries, entry))
                 goto err;
-            sk_X509_NAME_ENTRY_set(entries, j, NULL);
+            (void)sk_X509_NAME_ENTRY_set(entries, j, NULL);
         }
     }
     ret = x509_name_canon(nm.x);