wlan: Resolve invalid memory access while posting SYS_MSG_ID_MC_THR_PROBE

While posting SYS_MSG_ID_MC_THR_PROBE event, initialize callback variable
to NULL, to resolve accessing invalid memory. when MC thread process the
same request.

Change-Id: I8fc06f4b462a73eb584054f2ba2081824bc46520
CRs-Fixed: 936769
diff --git a/CORE/SYS/common/src/wlan_qct_sys.c b/CORE/SYS/common/src/wlan_qct_sys.c
index f55bd8c..be54dff 100644
--- a/CORE/SYS/common/src/wlan_qct_sys.c
+++ b/CORE/SYS/common/src/wlan_qct_sys.c
@@ -778,6 +778,7 @@
 
     vosMessage.reserved = FTM_SYS_MSG_COOKIE;
     vosMessage.type     = SYS_MSG_ID_MC_THR_PROBE;
+    vosMessage.callback = NULL;
     vosMessage.bodyptr  = NULL;
 
     vos_mq_post_message(VOS_MQ_ID_SYS, &vosMessage);