Fix EVP_DigestInit_ex with NULL digest

Calling EVP_DigestInit_ex which has already had the digest set up for it
should be possible. You are supposed to be able to pass NULL for the type.
However currently this seg faults.
Reviewed-by: NAndy Polyakov <appro@openssl.org>

crypto/evp/digest.c

@@ -187,9 +187,12 @@ int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl)
             ctx->engine = impl;
         } else
             ctx->engine = NULL;
-    } else if (!ctx->digest) {
-        EVPerr(EVP_F_EVP_DIGESTINIT_EX, EVP_R_NO_DIGEST_SET);
-        return 0;
+    } else {
+        if (!ctx->digest) {
+            EVPerr(EVP_F_EVP_DIGESTINIT_EX, EVP_R_NO_DIGEST_SET);
+            return 0;
+        }
+        type = ctx->digest;
     }
 #endif
     if (ctx->digest != type) {