Comment 7 for bug 1217158

A debug-only crash because the I_S function is calling buf_block_get_frame on any page it reads, which debug-asserts that the page is buffer-fixed, which is not the case in I_S query. Possible fixes: 1) acquire the buffer page mutex, buffer-fix the page, read the fields, buffer-unfix the page under the mutex; 2) just hold the buffer page mutex while the fields are read. The latter option seems more reasonable.