Fix indexing of M_.endo_trends

In the attached file, the indexing for the first variable C starts at 0 instead of 1

M_.endo_trends(0).deflator = 'A*L';

leading to a crash with

Array indices must be positive integers or logical values.

Ramsey_Cass_Koopmans_trend_var.mod

added bug label

assigned to @sebastien

closed with commit 2758f46c

mentioned in commit 17cfb42a