backend: guard prctl(PR_SET_NAME) against NULL thread name

o->comm may be NULL if job initialization fails or the job
structure is only partially initialized before thread creation.
Calling prctl(PR_SET_NAME, NULL) results in a NULL pointer
dereference inside strncpy().

Add a NULL check before calling prctl().

Reported-by: Criticayon Black
Signed-off-by: Criticayon Black <1318083585@qq.com>

Author: Criticayon

backend.c

@@ -1823,6 +1823,7 @@ static void *thread_main(void *data)
 	fio_local_clock_init();
 
 #ifdef CONFIG_LINUX
+if (o->comm)//fix:Add a simple NULL check
 	prctl(PR_SET_NAME, o->comm);
 #endif