azure: Pass data length to Azure libray

The azureAdapter was used directly without a pointer, but the Len()
method was only defined with a pointer receiver (which means Len() is
not present on a azureAdapter{}, only on a pointer to it).

internal/backend/azure/azure.go

@@ -123,7 +123,7 @@ type azureAdapter struct {
 
 func (azureAdapter) Close() error { return nil }
 
-func (a *azureAdapter) Len() int {
+func (a azureAdapter) Len() int {
 	return int(a.Length())
 }