Missing mutex != NULL check in i_s_innodb_fill_buffer_pool()

[Laurynas Biveinis]

Taking this from the MP review as it was submitted too late:

My previous review comment was misleading, sorry about that: buf_page_get_mutex_enter() is better than buf_page_get_mutex() and mutex_enter() combination by preventing a race condition between the calls and not calling mutex_enter on NULL. But it still might return NULL, thus mutex_exit() still has to be conditional in i_s_innodb_fill_buffer_pool().

[Laurynas Biveinis (laurynas-biveinis)]

And for 5.1:

i_s_innodb_fill_buffer_pool() still acquires the buffer pool mutex (and has a comment block above it).

Also, mutex_exit() in it should be conditional on block_mutex != NULL.