Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?

(N/A) While testing the Lassaigne's extract for the presence of halogens,it is first boiled with dilute nitric acid.
This is done to decompose $NaCN$ to $HCN$ and $Na_{2}S$ to $H_{2}S$ and to expel these gases.
If nitrogen and sulphur are present in the form of $NaCN$ and $Na_{2}S$,they would interfere with the test for halogens by forming $AgCN$ and $Ag_{2}S$ precipitates,which are also insoluble in water.
The chemical equations involved in the reaction are:
$NaCN + HNO_{3} \longrightarrow NaNO_{3} + HCN \uparrow$
$Na_{2}S + 2HNO_{3} \longrightarrow 2NaNO_{3} + H_{2}S \uparrow$