ospfd: Do not turn on write thread unless we have something in it

I am rarely seeing this crash:

r2: ospfd crashed. Core file found - Backtrace follows:
[New LWP 32748]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/aarch64-linux-gnu/libthread_db.so.1".
Core was generated by `/usr/lib/frr/ospfd'.
Program terminated with signal SIGABRT, Aborted.
2019-08-29 15:59:36,149 ERROR: assert failed at "test_ospf_sr_topo1/test_memory_leak":

Which translates to this code:

	node = listhead(ospf->oi_write_q);
	assert(node);
	oi = listgetdata(node);
	assert(oi);

So if we get into ospf_write without anything on the oi_write_q
we are stopping the program.

The only place that I see where we could possibly have this condition
was in OSPF_ISM_WRITE_ON(O).  Modify the macro to not just blindly
turn on ospf_write since we do not always add to the oi_write_q.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

ospfd/ospf_ism.h

@@ -53,8 +53,9 @@
 			listnode_add((O)->oi_write_q, oi);                     \
 			oi->on_write_q = 1;                                    \
 		}                                                              \
-		thread_add_write(master, ospf_write, (O), (O)->fd,             \
-				 &(O)->t_write);                               \
+		if (!list_isempty((O)->oi_write_q))                            \
+			thread_add_write(master, ospf_write, (O), (O)->fd,     \
+					 &(O)->t_write);                       \
 	} while (0)
 
 /* Macro for OSPF ISM timer turn on. */