[#17798]: Fix VerifyStream::Verify causes dereferencing an invalid pointer

Author: cchung100m

src/tir/analysis/verify_well_formed.cc

@@ -248,20 +248,26 @@ class UndefinedVarVerifier : public Verifier<UndefinedVarVerifier> {
     bool redefine_is_allowed = redefine_allowed_within_function_.count(var);
     {
       auto it = currently_defined_.find(var);
-      Verify(it == currently_defined_.end() || redefine_is_allowed)
-          << "ValueError: "
-          << "TIR is ill-formed, "
-          << "due to multiple nested definitions of variable " << var
-          << ".  It was first defined at " << it->second << ", and was re-defined at " << path;
+      auto verify = Verify(it == currently_defined_.end() || redefine_is_allowed);
+      verify << "ValueError: "
+             << "TIR is ill-formed, "
+             << "due to multiple nested definitions of variable " << var << ".";
+      if (it != currently_defined_.end()) {
+        verify << " It was first defined at " << it->second
+               << ", and was re-defined at " << path;
+      }
     }
 
     {
       auto it = previously_defined_.find(var);
-      Verify(it == previously_defined_.end() || redefine_is_allowed)
-          << "ValueError: "
-          << "TIR is ill-formed, "
-          << "due to multiple definitions of variable " << var << ".  It was first defined at "
-          << it->second << ", and was later re-defined at " << path;
+      auto verify = Verify(it == previously_defined_.end() || redefine_is_allowed);
+      verify << "ValueError: "
+             << "TIR is ill-formed, "
+             << "due to multiple definitions of variable " << var << ".";
+      if (it != previously_defined_.end()) {
+        verify << " It was first defined at " << it->second
+               << ", and was later re-defined at " << path;
+      }
     }
 
     currently_defined_.insert({var, path});