Resolve Coverity complaint in storagevolxml2argvtest

Ignore NULL pool in testSetVolumeType to silence Coverity,
even though we only call it with NULL pool when vol is also NULL.

(13) Event var_deref_model:	Passing null pointer "inputpool" to
function "testSetVolumeType(virStorageVolDefPtr, virStoragePoolDefPtr)",
which dereferences it. [details]
Also see events: [assign_zero]

95    testSetVolumeType(inputvol, inputpool);

tests/storagevolxml2argvtest.c

@@ -16,7 +16,7 @@ static void
 testSetVolumeType(virStorageVolDefPtr vol,
                   virStoragePoolDefPtr pool)
 {
-    if (!vol)
+    if (!vol || !pool)
         return;
 
     switch (pool->type) {