Initialise message_handler in bv_refinement unit test

The message_handler member of bv_refinementt::infot was left
uninitialised, which is undefined behaviour when it is later
dereferenced. Set it to null_message_handler.

Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>

Author: tautschnig

jbmc/unit/solvers/strings/string_constraint_instantiation/instantiate_not_contains.cpp

@@ -171,6 +171,7 @@ decision_proceduret::resultt check_sat(const exprt &expr, const namespacet &ns)
   bv_refinementt::infot info;
   info.ns = &ns;
   info.prop = &sat_check;
+  info.message_handler = &null_message_handler;
   info.output_xml = false;
   bv_refinementt solver(info);
   solver << expr;