mkList: Scrub better

Clearing v.app.right was not enough, because the length field of a
list only takes 32 bits, so the most significant 32 bits of v.app.left
(a.k.a. v.thunk.env) would remain. This could cause Boehm GC to
interpret it as a valid pointer.

This change reduces maximum RSS for evaluating the ‘tested’ job in
nixos/release-small.nix from 1.33 GiB to 0.80 GiB, and runtime by
about 8%.

src/libexpr/eval.cc

@@ -455,6 +455,7 @@ Bindings * EvalState::allocBindings(Bindings::size_t capacity)
 
 void EvalState::mkList(Value & v, unsigned int length)
 {
+    clearValue(v);
     v.type = tList;
     v.list.length = length;
     v.list.elems = length ? (Value * *) GC_MALLOC(length * sizeof(Value *)) : 0;

src/libexpr/value.hh

@@ -96,7 +96,7 @@ struct Value
    Value to ensure that the target isn't kept alive unnecessarily. */
 static inline void clearValue(Value & v)
 {
-    v.app.right = 0;
+    v.app.left = v.app.right = 0;
 }
 
 
@@ -118,8 +118,8 @@ static inline void mkBool(Value & v, bool b)
 
 static inline void mkNull(Value & v)
 {
+    clearValue(v);
     v.type = tNull;
-    v.app.left = v.app.right = 0; // scrub
 }