fix bug #!2231 about wrong typing of casts

src/plugins/e-acsl/tests/bts/bts2231.i

src/plugins/e-acsl/tests/bts/oracle/bts2231.err.oracle

src/plugins/e-acsl/tests/bts/oracle/bts2231.res.oracle

src/plugins/e-acsl/tests/bts/oracle/gen_bts1390.c

src/plugins/e-acsl/tests/bts/oracle/gen_bts2231.c

src/plugins/e-acsl/tests/runtime/oracle/gen_linear_search.c

src/plugins/e-acsl/tests/runtime/oracle/gen_ptr.c

src/plugins/e-acsl/typing.ml