[3.11] gh-105375: Improve error handling in compiler_enter_scope() (#105494) (#105582)

(cherry picked from commit 6c832dd)

Co-authored-by: Erlend E. Aasland <[email protected]>

Author: erlend-aasland

Misc/NEWS.d/next/Core and Builtins/2023-06-08-09-54-37.gh-issue-105375.kqKT3E.rst

@@ -0,0 +1 @@
+Fix bug in the compiler where an exception could end up being overwritten.

Python/compile.c

@@ -1736,8 +1736,12 @@ compiler_enter_scope(struct compiler *c, identifier name,
     Py_INCREF(name);
     u->u_name = name;
     u->u_varnames = list2dict(u->u_ste->ste_varnames);
+    if (!u->u_varnames) {
+        compiler_unit_free(u);
+        return 0;
+    }
     u->u_cellvars = dictbytype(u->u_ste->ste_symbols, CELL, 0, 0);
-    if (!u->u_varnames || !u->u_cellvars) {
+    if (!u->u_cellvars) {
         compiler_unit_free(u);
         return 0;
     }