compiler: Fix minor codegen issues after pickling

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (2a9533f) 86.80% compared to head (9ee3471) 86.86%.

Files	Patch %	Lines
devito/operator/operator.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2283 +/- ## ========================================== + Coverage 86.80% 86.86% +0.05% ========================================== Files 229 229 Lines 42516 42535 +19 Branches 7889 7892 +3 ========================================== + Hits 36908 36949 +41 + Misses 4952 4929 -23 - Partials 656 657 +1 ```

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