Fix ABI break with HMAC

Recent HMAC changes broke ABI compatibility due to a new field in HMAC_CTX.
This backs that change out, and does it a different way.

Thanks to Timo Teras for the concept.

Reviewed-by: Richard Levitte <levitte@openssl.org>

test/hmactest.c

diff --git a/test/hmactest.c b/test/hmactest.c
index 13344d6..a9b829d 100644
--- a/test/hmactest.c
+++ b/test/hmactest.c
@@ -226,7 +226,12 @@
         err++;
         goto test6;
     }
-    if (!HMAC_Init_ex(&ctx, NULL, 0, EVP_sha256(), NULL)) {
+    if (HMAC_Init_ex(&ctx, NULL, 0, EVP_sha256(), NULL)) {
+        printf("Should disallow changing MD without a new key (test 5)\n");
+        err++;
+        goto test6;
+    }
+    if (!HMAC_Init_ex(&ctx, test[4].key, test[4].key_len, EVP_sha256(), NULL)) {
         printf("Failed to reinitialise HMAC (test 5)\n");
         err++;
         goto test6;