What guarantees that endowed agents have non-zero prices in an Arrow-Debreu Economy
A pretty trivial assumption would do it:

If the consumers preferences are strongly increasing, i.e. 
$$
x \succ_i y, \mbox{  if $x \geq y$ but $x \neq y$ },
$$
and unbounded for all $i$, then no $x$ can be an optimal bundle if $p \notin \mathbb{R}^C_{++}$ (assuming there are $C$ commodities).
So zero price cannot arise in an Arrow-Debreu equilibrium.