[fix] call pprzlog_init if needed (and avoid segfault)

conf/firmwares/subsystems/shared/sdlog.makefile

@@ -1,6 +1,6 @@
 # Hey Emacs, this is a -*- makefile -*-
 
-sdlog_CFLAGS = -DDOWNLINK
+sdlog_CFLAGS = -DDOWNLINK -DUSE_PPRZLOG
 sdlog_srcs = subsystems/datalink/downlink.c subsystems/datalink/pprzlog_transport.c
 
 ap.CFLAGS += $(sdlog_CFLAGS)

sw/airborne/subsystems/datalink/downlink.c

@@ -75,6 +75,10 @@ void downlink_init(void)
 #endif
 #endif
 
+#if USE_PPRZLOG
+  pprzlog_transport_init();
+#endif
+
 #if SITL
   ivy_transport_init();
 #endif

sw/airborne/subsystems/datalink/pprzlog_transport.h

@@ -41,5 +41,8 @@ struct pprzlog_transport {
 
 extern struct pprzlog_transport pprzlog_tp;
 
+// Init function
+extern void pprzlog_transport_init(void);
+
 #endif