Fix a potential double free in EVP_DigestInit_ex

There is a potential double free in EVP_DigestInit_ex. This is believed
to be reached only as a result of programmer error - but we should fix it
anyway.

Issue reported by Guido Vranken.

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

crypto/evp/digest.c

@@ -219,8 +219,10 @@ int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl)
     }
 #endif
     if (ctx->digest != type) {
-        if (ctx->digest && ctx->digest->ctx_size)
+        if (ctx->digest && ctx->digest->ctx_size) {
             OPENSSL_free(ctx->md_data);
+            ctx->md_data = NULL;
+        }
         ctx->digest = type;
         if (!(ctx->flags & EVP_MD_CTX_FLAG_NO_INIT) && type->ctx_size) {
             ctx->update = type->update;