Implement coverity suggestions (#2730)

### Issues:
Addresses P311843643

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under the terms of the Apache 2.0 license and the ISC license.

Author: skmcgrail

tool-openssl/dgst.cc

@@ -113,9 +113,11 @@ static std::string GetSigName(int nid) {
     case EVP_PKEY_EC:
       return "ECDSA";
 
-    default:
+    default: {
       /* Try to output provider-registered sig alg name */
-      return OBJ_nid2sn(nid);
+      const char* name = OBJ_nid2sn(nid);
+      return name ? std::string(name) : "UNKNOWN";
+    }
   }
 }
 

tool-openssl/ordered_args.cc

@@ -149,7 +149,7 @@ bool GetBoolArgument(bool *out, const std::string &arg_name,
 bool GetExclusiveBoolArgument(std::string *out_arg, const argument_t *templates,
                               std::string default_out_arg,
                               const ordered_args_map_t &args) {
-  *out_arg = default_out_arg;
+  *out_arg = std::move(default_out_arg);
 
   for (size_t i = 0; templates[i].name[0] != 0; i++) {
     const argument_t *templ = &templates[i];