[cli] Ensure that probed libstdc++ path is NULL-terminated

It appears that we were assuming our path was initialized with zeros, but that is not a safe assumption.

cli/loader_lib.c

@@ -345,6 +345,8 @@ static char *libstdcxxprobe(void)
             free(path);
             return NULL;
         }
+        // Ensure that `path` is zero-terminated.
+        path[pathlen] = '\0';
         return path;
     }
 }