Don't look inside cursor for a session pointer if we already have one.

Fixes:
*** CID 1203474:  Explicit null dereferenced  (FORWARD_NULL)
/src/lsm/lsm_cursor.c: 1360 in __wt_clsm_open()
   Dereferencing null pointer "cursor".
1360     		WT_RET_MSG((WT_SESSION_IMPL *)cursor->session, EINVAL,
Fix the "LSM cannot open a checkpoint" error message.