add missing null-termination

mrbgems/mruby-bin-debugger/tools/mrdb/apilist.c

@@ -75,6 +75,7 @@ dirname(mrb_state *mrb, const char *path)
 
   if ((dir = mrb_malloc(mrb, len + 1)) != NULL) {
     strncpy(dir, path, len);
+    dir[len] = '\0';
   }
   return dir;
 }