virGetStorageVol: Don't ignore NULL pool name

The function takes pool name as argument. However,
it is not acceptable for it to be NULL. Hence, we
should check it and report error in case it is.

src/datatypes.c

@@ -520,6 +520,7 @@ virGetStorageVol(virConnectPtr conn, const char *pool, const char *name,
         virLibConnError(VIR_ERR_INVALID_CONN, "%s", _("no connection"));
         return NULL;
     }
+    virCheckNonNullArgReturn(pool, NULL);
     virCheckNonNullArgReturn(name, NULL);
     virCheckNonNullArgReturn(key, NULL);