util: don't validate empty params

If there are no parameters, there is nothing to validate.
If params == NULL, memcpy below results in memcpy(sorted, NULL, 0),
which is UB.

Found by UBSAN. Example of this codepath: virDomainBlockCopy()
(where nparams == 0 is valid) -> qemuDomainBlockCopy()

Signed-off-by: Oleg Vasilev <oleg.vasilev@virtuozzo.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>

src/util/virtypedparam.c

@@ -68,6 +68,10 @@ virTypedParamsValidate(virTypedParameterPtr params, int nparams, ...)
     g_autofree virTypedParameterPtr sorted = NULL;
     g_autofree virTypedParameterPtr keys = NULL;
 
+    if (!nparams) {
+        return 0;
+    }
+
     va_start(ap, nparams);
 
     sorted = g_new0(virTypedParameter, nparams);