Extract unsigned value from options

Author: tautschnig

src/cbmc/cbmc_solvers.cpp

@@ -145,7 +145,8 @@ std::unique_ptr<cbmc_solverst::solvert> cbmc_solverst::get_string_refinement()
   info.refinement_bound=DEFAULT_MAX_NB_REFINEMENT;
   info.ui=ui;
   if(options.get_bool_option("string-max-length"))
-    info.max_string_length = options.get_signed_int_option("string-max-length");
+    info.max_string_length =
+      options.get_unsigned_int_option("string-max-length");
   info.trace=options.get_bool_option("trace");
   if(options.get_bool_option("max-node-refinement"))
     info.max_node_refinement=