Fix a leak on the fine-grained-durability branch:

==42696== 983,040 bytes in 15 blocks are definitely lost in loss record 1 of 1
==42696==    at 0x4A0887C: malloc (vg_replace_malloc.c:270)
==42696==    by 0x4A089F0: realloc (vg_replace_malloc.c:662)
==42696==    by 0x4CA068B: __wt_realloc (os_alloc.c:80)
==42696==    by 0x4CC0B26: __wt_buf_grow (scratch.c:69)
==42696==    by 0x4CC0BE6: __wt_buf_init (scratch.c:102)
==42696==    by 0x4C8FB89: __wt_log_slot_init (log_slot.c:57)
==42696==    by 0x4C7522A: __wt_logmgr_create (conn_log.c:187)
==42696==    by 0x4C76057: __wt_connection_workers (conn_open.c:212)
==42696==    by 0x4C7038F: wiredtiger_open (conn_api.c:1185)
==42696==    by 0x4008A8: main (ex_access.c:46)