rate_ctr: Fix null pointer dereference in error path

In Change-Id Ifc6ac824f5dae9a848bb4a5d067c64a69eb40b56 we introduced
a variable de-reference before we check if it's NULL.

Let's reorder the statements to avoid this.

Fixes: Coverity CID#178219
Change-Id: I99265a7ee76f85c479543c19ce8c05ce5d43ae69
diff --git a/src/rate_ctr.c b/src/rate_ctr.c
index 6de59a0..296cc16 100644
--- a/src/rate_ctr.c
+++ b/src/rate_ctr.c
@@ -74,12 +74,13 @@
 static bool rate_ctrl_group_desc_validate(const struct rate_ctr_group_desc *desc, bool quiet)
 {
 	unsigned int i;
-	const struct rate_ctr_desc *ctr_desc = desc->ctr_desc;
+	const struct rate_ctr_desc *ctr_desc;
 
 	if (!desc) {
 		LOGP(DLGLOBAL, LOGL_ERROR, "NULL is not a valid counter group descriptor\n");
 		return false;
 	}
+	ctr_desc = desc->ctr_desc;
 
 	DEBUGP(DLGLOBAL, "validating counter group %p(%s) with %u counters\n", desc,
 		desc->group_name_prefix, desc->num_ctr);