Check for BN_copy failures.

BN_copy can fail on malloc failure. The case in crypto/rsa was causing the
malloc tests in all_tests.go to infinite loop.

Change-Id: Id5900512013fba9960444d78a8c056aa4314fb2d
Reviewed-on: https://boringssl-review.googlesource.com/5110
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/bn.h

diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index ec1c8ff..a61e5cf 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -182,7 +182,8 @@
  * allocated BIGNUM on success or NULL otherwise. */
 OPENSSL_EXPORT BIGNUM *BN_dup(const BIGNUM *src);
 
-/* BN_copy sets |dest| equal to |src| and returns |dest|. */
+/* BN_copy sets |dest| equal to |src| and returns |dest| or NULL on allocation
+ * failure. */
 OPENSSL_EXPORT BIGNUM *BN_copy(BIGNUM *dest, const BIGNUM *src);
 
 /* BN_clear sets |bn| to zero and erases the old data. */