promql/parser: Cleanup generatedParserResult accross reuse
promql/parser: Cleanup generatedParserResult accross reuse

Reusing the same generatedParserResult ends up in strange panics:
See #7131 and #7127.

Signed-off-by: Julien Pivotto <df1ed88a312a603dfee9f157a90fa8f7f195ab59@inuits.eu>

GoogleCloudPlatform/prometheus,kangwoo/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,kangwoo/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,alileza/prometheus,kangwoo/prometheus,alileza/prometheus