Update local_scan.c

Update local_scan.c

[../rspamd-master/contrib/exim/local_scan.c:630]: (error) Uninitialized variable: rej

It's always good to initialize integer variables, at least to 0, because if you try to retrieve its value before it gets assigned any actual (non-garbage) value, then it results in undefined behavior.

Found by https://github.com/bryongloden/cppcheck