8297874: get_dump_directory() in jfrEmergencyDump.cpp should pass correct length to jio_snprintf

Reviewed-by: mgronlun

src/hotspot/share/jfr/recorder/repository/jfrEmergencyDump.cpp

@@ -78,7 +78,7 @@ static size_t get_dump_directory() {
   }
   const size_t path_len = strlen(_path_buffer);
   const int result = jio_snprintf(_path_buffer + path_len,
-                                  sizeof(_path_buffer),
+                                  sizeof(_path_buffer) - path_len,
                                   "%s",
                                   os::file_separator());
   return (result == -1) ? 0 : strlen(_path_buffer);