fellow_log: Polish happy bitfield update

With the previous code, the compiler could have produced machine code
which produced a transient zero lsb when in fact it should have been 1.

Ref ba32426c
Ref #66

src/fellow_log.c

@@ -716,6 +716,7 @@ void
 fellow_fd_update_stats(struct fellow_fd *ffd)
 {
 	buddy_t *buddy;
+	uint64_t happy;
 	size_t sz;
 
 	CHECK_OBJ_NOTNULL(ffd, FELLOW_FD_MAGIC);
@@ -735,8 +736,10 @@ fellow_fd_update_stats(struct fellow_fd *ffd)
 	ffd->stats->g_mem_space = sz;
 	ffd->stats->g_mem_bytes = buddy_size(buddy) - sz;
 
-	ffd->stats->b_happy <<= 1;
-	ffd->stats->b_happy |= !! fellow_is_open(ffd);	//lint !e514
+	happy = ffd->stats->b_happy;
+	happy <<= 1;
+	happy |= !! fellow_is_open(ffd);	//lint !e514
+	ffd->stats->b_happy = happy;
 }
 
 static void