evp_pmeth: free the MD reference correctly.

The code was calling EVP_MD_meth_free which is incorrect.  It should call
EVP_MD_free.  It happened to work but by luck rather than design.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10973)

crypto/evp/pmeth_lib.c

@@ -834,7 +834,7 @@ static int legacy_ctrl_str_to_param(EVP_PKEY_CTX *ctx, const char *name,
         if (md == NULL)
             return 0;
         ret = EVP_PKEY_CTX_set_signature_md(ctx, md);
-        EVP_MD_meth_free(md);
+        EVP_MD_free(md);
         return ret;
     }