Coverity analysis defect 99861: Dereference after null check

Pointer is checked against null but then dereferenced anyway

Defect 99861 (STATIC_C)
Checker FORWARD_NULL (subcategory none)
File: /src/third_party/wiredtiger/src/os_posix/os_mtx_cond.c
Function __wt_cond_wait_signal
/src/third_party/wiredtiger/src/os_posix/os_mtx_cond.c, line: 67
Comparing "session" to null implies that "session" might be null.

    	if (session != NULL) {