Fix a segmentation fault caused by a negative argc

On some platforms the cmd_line_args function sets cfg::largc to -1 and
trigger a segmentation fault in parse.

include/boost/ut.hpp

@@ -803,7 +803,7 @@ struct cfg {
   }
 
   static inline void parse(int argc, const char* argv[]) {
-    const std::size_t n_args = static_cast<std::size_t>(argc);
+    const std::size_t n_args = argc > 0 ? static_cast<std::size_t>(argc) : 0U;
     if (n_args > 0 && argv != nullptr) {
       cfg::largc = argc;
       cfg::largv = argv;