Fix GH 327.

Valgrind complains about using unitialized memory.  So call
OPENSSL_zalloc, not malloc.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/evp/digest.c

@@ -229,7 +229,7 @@ int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl)
         ctx->digest = type;
         if (!(ctx->flags & EVP_MD_CTX_FLAG_NO_INIT) && type->ctx_size) {
             ctx->update = type->update;
-            ctx->md_data = OPENSSL_malloc(type->ctx_size);
+            ctx->md_data = OPENSSL_zalloc(type->ctx_size);
             if (ctx->md_data == NULL) {
                 EVPerr(EVP_F_EVP_DIGESTINIT_EX, ERR_R_MALLOC_FAILURE);
                 return 0;