gh-105375: Improve error handling in compiler_enter_scope() (#105494)

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

@@ -1228,8 +1228,12 @@ compiler_enter_scope(struct compiler *c, identifier name,
     }
     u->u_metadata.u_name = Py_NewRef(name);
     u->u_metadata.u_varnames = list2dict(u->u_ste->ste_varnames);
+    if (!u->u_metadata.u_varnames) {
+        compiler_unit_free(u);
+        return ERROR;
+    }
     u->u_metadata.u_cellvars = dictbytype(u->u_ste->ste_symbols, CELL, DEF_COMP_CELL, 0);
-    if (!u->u_metadata.u_varnames || !u->u_metadata.u_cellvars) {
+    if (!u->u_metadata.u_cellvars) {
         compiler_unit_free(u);
         return ERROR;
     }