Don't show "from call site" when we don't know the call site

This gets rid of stack trace entries like

       … from call site

       at «stdin»:0: (source not available)

src/libexpr/eval.cc

@@ -1617,7 +1617,8 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                         (lambda.name
                             ? concatStrings("'", symbols[lambda.name], "'")
                             : "anonymous lambda"));
-                    addErrorTrace(e, pos, "from call site%s", "");
+                    if (pos != noPos)
+                        addErrorTrace(e, pos, "from call site", "");
                 }
                 throw;
             }

src/libexpr/nixexpr.hh

@@ -53,6 +53,10 @@ public:
     explicit operator bool() const { return id > 0; }
 
     bool operator <(const PosIdx other) const { return id < other.id; }
+
+    bool operator ==(const PosIdx other) const { return id == other.id; }
+
+    bool operator !=(const PosIdx other) const { return id != other.id; }
 };
 
 class PosTable