If the context already have a EVP_MD_CTX allocated, don't make a new

Patch from Derrick Pallas of Riverbed Techology

lib/hcrypto/hmac.c

@@ -121,6 +121,7 @@ HMAC_Init_ex(HMAC_CTX *ctx,
     for (i = 0, p = ctx->opad; i < keylen; i++)
 	p[i] ^= ((const unsigned char *)key)[i];
 
+    if (ctx->ctx == NULL)
 	ctx->ctx = EVP_MD_CTX_create();
 
     EVP_DigestInit_ex(ctx->ctx, ctx->md, ctx->engine);