[3.12] gh-120155: Fix optimize_and_assemble_code_unit() error handling (#120231)

gh-120155: Fix optimize_and_assemble_code_unit() error handling

Don't use 'g' before it's being initialized: don't use the 'error'
label if consts_dict_keys_inorder() failed.

Fix the Coverity issue:

Error: UNINIT (CWE-457):
Python-3.12.2/Python/compile.c:7670:5: skipped_decl: Jumping over declaration of ""g"".
Python-3.12.2/Python/compile.c:7714:5: uninit_use_in_call: Using uninitialized value ""g.g_block_list"" when calling ""_PyCfgBuilder_Fini"".
Python-3.12.2/Python/compile.c:7714:5: uninit_use_in_call: Using uninitialized value ""g.g_entryblock"" when calling ""_PyCfgBuilder_Fini"".
  7712|       Py_XDECREF(consts);
  7713|       instr_sequence_fini(&optimized_instrs);
  7714|->     _PyCfgBuilder_Fini(&g);
  7715|       return co;
  7716|   }