Clear md_data only when necessary

PR #17255 fixed a bug in EVP_DigestInit_ex(). While backporting the PR
to 1.1.1 (see #17472) I spotted an error in the original patch. This fixes
it.
Reviewed-by: NTomas Mraz <tomas@openssl.org>
Reviewed-by: NPaul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17473)

crypto/evp/digest.c

@@ -33,9 +33,10 @@ static void cleanup_old_md_data(EVP_MD_CTX *ctx, int force)
             ctx->digest->cleanup(ctx);
         if (ctx->md_data != NULL && ctx->digest->ctx_size > 0
                 && (!EVP_MD_CTX_test_flags(ctx, EVP_MD_CTX_FLAG_REUSE)
-                    || force))
+                    || force)) {
             OPENSSL_clear_free(ctx->md_data, ctx->digest->ctx_size);
-        ctx->md_data = NULL;
+            ctx->md_data = NULL;
+        }
     }
 }