Add error-check for yield-in-pcall to traceback_pcall

luprex/core/cpp/traceback.cpp

@@ -1,6 +1,7 @@
 #include "traceback.hpp"
 
 #include <cstring>
+#include <cassert>
 
 #define TRACEBACK_LEVELS1 12
 #define TRACEBACK_LEVELS2 10
@@ -88,7 +89,9 @@ eng::string traceback_pcall(lua_State *L, int narg, int nret) {
         if ((msg == NULL) || (msg[0] == 0)) {
             msg = "unknown error";
         }
-        return msg;
+        eng::string result = msg;
+        assert(result != "attempt to yield from outside a coroutine");
+        return result;
     }
     return "";
 }