fix get_vesting_balance method

libraries/chain/vesting_balance_object.cpp

@@ -44,6 +44,15 @@ asset linear_vesting_policy::get_allowed_withdraw( const vesting_policy_context&
         assert( elapsed_seconds > 0 );
 
         if( elapsed_seconds >= vesting_cliff_seconds )
+        {
+            // BLOCKBACK-154 fix, Begin balance for linear vesting applies only to initial account balance from genesis
+            // So, for any GPOS vesting, the begin balance would be 0 and should be able to withdraw balance amount based on lockin period
+            if(begin_balance == 0)
+            {
+               allowed_withdraw = ctx.balance.amount;
+               return asset( allowed_withdraw, ctx.balance.asset_id );
+            }
+            else
             {
                share_type total_vested = 0;
                if( elapsed_seconds < vesting_duration_seconds )
@@ -63,6 +72,7 @@ asset linear_vesting_policy::get_allowed_withdraw( const vesting_policy_context&
                assert( allowed_withdraw >= 0 );
             }
          }
+    }
 
     return asset( allowed_withdraw, ctx.balance.asset_id );
 }