requester: Add null check in pldm_instance_id_alloc

pldm_instance_id_alloc() function does not check for a valid
ctx object before dereferencing it. Hence it could cause
crash when its clients pass a null pointer by accident.

Change-Id: I340aa8171cd397f5af772a9cc6d4f80c8263a089
Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
diff --git a/src/requester/instance-id.c b/src/requester/instance-id.c
index d891a48..10dc276 100644
--- a/src/requester/instance-id.c
+++ b/src/requester/instance-id.c
@@ -122,7 +122,7 @@
 {
 	uint8_t l_iid;
 
-	if (!iid) {
+	if (!ctx || !iid) {
 		return -EINVAL;
 	}