Lexical envs: fix inverted logic

In exception handlers, we must free resources when it *is* a "legit"
exception taht is raised, and leave them unchanged in other cases. This
fixes a memory leak in the former case.

For #632

langkit/support/langkit_support-lexical_envs_impl.adb

@@ -1408,7 +1408,7 @@ package body Langkit_Support.Lexical_Envs_Impl is
          --  All bets are off for other kinds of exceptions: just propagate in
          --  that case.
 
-         if not Properties_May_Raise (Exc) then
+         if Properties_May_Raise (Exc) then
             Local_Results.Destroy;
          end if;