Test program: Don't issue errors when the value cannot be read
Test program: Don't issue errors when the value cannot be read

Soon, the XML backend functions to read/write attributes will be
dropped. This commit ensures that the test program does not display
errors when the XML backend is used.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

Sunderfield/libiio,analogdevicesinc/libiio,sensarliar/libiio,sensarliar/libiio,gburca/libiio,gburca/libiio,gburca/libiio,Sunderfield/libiio,sensarliar/libiio,analogdevicesinc/libiio,Sunderfield/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio