fixed link secret unwrap failure (#314)

Author: rytmarsh

vcx/libvcx/src/disclosed_proof.rs

@@ -42,7 +42,7 @@ impl Default for DisclosedProof {
             state: VcxStateType::VcxStateNone,
             proof_request: None,
             proof: None,
-            link_secret_alias: settings::get_config_value(settings::CONFIG_LINK_SECRET_ALIAS).unwrap(),
+            link_secret_alias: settings::DEFAULT_LINK_SECRET_ALIAS.to_string(),
             their_did: None,
             their_vk: None,
             agent_did: None,

vcx/libvcx/src/utils/libindy/anoncreds.rs

@@ -307,7 +307,7 @@ pub fn libindy_prover_create_credential_req(prover_did: &str,
     let prover_did = CString::new(prover_did).map_err(map_string_error)?;
     let credential_offer_json = CString::new(credential_offer_json).map_err(map_string_error)?;
     let credential_def_json = CString::new(credential_def_json).map_err(map_string_error)?;
-    let master_secret_name = CString::new(master_secret_id.unwrap_or(settings::get_config_value(settings::CONFIG_LINK_SECRET_ALIAS).unwrap())).map_err(map_string_error)?;
+    let master_secret_name = CString::new(master_secret_id.unwrap_or(settings::DEFAULT_LINK_SECRET_ALIAS.to_string())).map_err(map_string_error)?;
 
     unsafe {
         indy_function_eval(