libexpr: Make unsafeGetAttrPos not crash on noPos

Currently e.g. `builtins.unsafeGetAttrPos "abort" builtins` will
eventually segfault because pos->file is an unset Symbol.

Found by afl-fuzz.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 37b977736e28..8548990795ab 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -628,7 +628,7 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 
 void EvalState::mkPos(Value & v, Pos * pos)
 {
-    if (pos) {
+    if (pos && pos->file.set()) {
         mkAttrs(v, 3);
         mkString(*allocAttr(v, sFile), pos->file);
         mkInt(*allocAttr(v, sLine), pos->line);