[wp] fix errors in stats

src/plugins/wp/Cache.ml

src/plugins/wp/ProofEngine.ml

src/plugins/wp/ProverScript.ml

src/plugins/wp/Stats.ml

src/plugins/wp/register.ml

...gins/wp/tests/wp_plugin/oracle_qualif/unsigned.res.oracle

src/plugins/wp/tests/wp_tip/oracle/clear.res.oracle

...lugins/wp/tests/wp_tip/oracle/induction_typing.res.oracle

src/plugins/wp/tests/wp_tip/oracle/split.res.oracle

...gins/wp/tests/wp_tip/oracle_qualif/induction.1.res.oracle

...gins/wp/tests/wp_tip/oracle_qualif/induction.2.res.oracle

...ugins/wp/tests/wp_tip/oracle_qualif/strategy.0.res.oracle

...ugins/wp/tests/wp_tip/oracle_qualif/strategy.1.res.oracle

...ugins/wp/tests/wp_tip/oracle_qualif/strategy.2.res.oracle

...ugins/wp/tests/wp_tip/oracle_qualif/strategy.3.res.oracle

...sts/wp_tip/oracle_qualif/tac_split_quantifiers.res.oracle