BUG/MINOR: log/encode: fix potential NULL-dereference in LOGCHAR()

When CBOR encoding was added in c614fd3b9 ("MINOR: log: add +cbor encoding
option"), in LOGCHAR(), we forgot to check that we don't assign the NULL
value to tmplog (as we assume that tmplog cannot be NULL at the end of
sess_build_logline())

No backport needed.

src/log.c

@@ -3163,6 +3163,8 @@ const char sess_set_cookie[8] = "NPDIRU67";	/* No set-cookie, Set-cookie found a
 				                       tmplog,                 \
 				                       dst + maxsize,          \
 				                       _x, sizeof(_x));        \
+				if (ret == NULL)                               \
+					goto out;                              \
 				tmplog = ret;                                  \
 				break;                                         \
 			}                                                      \