Fix typo in |HMAC_CTX_cleanup|.

This was part of https://boringssl-review.googlesource.com/#/c/2710,
but that got lost because I was gumption-trapped by the number of
changes in x509/ that I didn't really want to make.

Change-Id: Iaf5bc8bcc2e3cfbb1b37aa477462ee8f824135db
Reviewed-on: https://boringssl-review.googlesource.com/5440
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/hmac/hmac.c b/crypto/hmac/hmac.c
index 556e7f9..d37a249 100644
--- a/crypto/hmac/hmac.c
+++ b/crypto/hmac/hmac.c
@@ -97,7 +97,7 @@
   EVP_MD_CTX_cleanup(&ctx->i_ctx);
   EVP_MD_CTX_cleanup(&ctx->o_ctx);
   EVP_MD_CTX_cleanup(&ctx->md_ctx);
-  OPENSSL_cleanse(ctx, sizeof(ctx));
+  OPENSSL_cleanse(ctx, sizeof(HMAC_CTX));
 }
 
 int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, size_t key_len,