Clean up data-flow in resolveVAddr.

Initialise the 'ret' struct from a constant zero struct to ensure that no
uninitialised data is returned. Returning uninitialized data creates headaches
for binary verification.

This function is inlined in builds I have looked at, and the 'ret' struct is
never built, so this change has zero impact on the binary.

src/arch/arm/kernel/vspace.c

@@ -1677,11 +1677,13 @@ decodeARMFrameInvocation(word_t label, unsigned int length,
     }
 }
 
+static const resolve_ret_t default_resolve_ret_t;
+
 static resolve_ret_t
 resolveVAddr(pde_t *pd, vptr_t vaddr)
 {
     pde_t *pde = lookupPDSlot(pd, vaddr);
-    resolve_ret_t ret;
+    resolve_ret_t ret = default_resolve_ret_t;
 
     ret.valid = true;