Clear key_method in ssl_cert_clear_certs.

Since it resets leaf, private key, and chain, it makes sense to also
clear custom key method tables.

Change-Id: If511b8f15a44674c31d068d36984e9189c5a9071
Reviewed-on: https://boringssl-review.googlesource.com/5356
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index afa4cec..50842e7 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -274,6 +274,7 @@
   cert->privatekey = NULL;
   sk_X509_pop_free(cert->chain, X509_free);
   cert->chain = NULL;
+  cert->key_method = NULL;
 }
 
 void ssl_cert_free(CERT *c) {