[wp] Improves null (in)validity

src/plugins/wp/MemMemory.ml

src/plugins/wp/share/why3/frama_c_wp/memory.mlw

src/plugins/wp/tests/wp_acsl/invalid_pointer.c

src/plugins/wp/tests/wp_acsl/null.c

src/plugins/wp/tests/wp_acsl/null.i

...lugins/wp/tests/wp_acsl/oracle/invalid_pointer.res.oracle

src/plugins/wp/tests/wp_acsl/oracle/null.res.oracle

...wp/tests/wp_acsl/oracle_qualif/invalid_pointer.res.oracle

src/plugins/wp/tests/wp_acsl/oracle_qualif/null.res.oracle