mac80211: allow null chandef in tracing

In TDLS channel-switch operations the chandef can sometimes be NULL.
Avoid an oops in the trace code for these cases and just print a
chandef full of zeros.

Cc: [e-mail]
Fixes: a7a6bdd0670fe ("mac80211: introduce TDLS channel switch ops")
Signed-off-by: Arik Nemtsov <[e-mail]>
Signed-off-by: Emmanuel Grumbach <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/trace.h

···
 			__field(u32, chan_width)					\
 			__field(u32, center_freq1)					\
 			__field(u32, center_freq2)
-#define CHANDEF_ASSIGN(c)								\
-			__entry->control_freq = (c)->chan ? (c)->chan->center_freq : 0;	\
-			__entry->chan_width = (c)->width;				\
-			__entry->center_freq1 = (c)->center_freq1;			\
-			__entry->center_freq2 = (c)->center_freq2;
+#define CHANDEF_ASSIGN(c)							\
+			__entry->control_freq = (c) ? ((c)->chan ? (c)->chan->center_freq : 0) : 0;	\
+			__entry->chan_width = (c) ? (c)->width : 0;			\
+			__entry->center_freq1 = (c) ? (c)->center_freq1 : 0;		\
+			__entry->center_freq2 = (c) ? (c)->center_freq2 : 0;
 #define CHANDEF_PR_FMT	" control:%d MHz width:%d center: %d/%d MHz"
 #define CHANDEF_PR_ARG	__entry->control_freq, __entry->chan_width,			\
 			__entry->center_freq1, __entry->center_freq2