pki_mbedcrypto: Use explicit_bzero()

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit b6b5a61c97)

src/pki_mbedcrypto.c

@@ -1046,7 +1046,7 @@ static ssh_string rsa_do_sign_hash(const unsigned char *digest,
     }
 
     ssh_string_fill(sig_blob, sig, slen);
-    memset(sig, 'd', slen);
+    explicit_bzero(sig, slen);
     SAFE_FREE(sig);
 
     return sig_blob;