Fix uninitialized indicators for conversions

```soci::indicator``` members of ```conversion_into_type``` and ```conversion_use_type``` specialisations are not value or zero-initialized, and get random values.

Fixes #152

**NOTE:** It needs to be decided if this fix should be released as hotfix in SOCI 3.2.2, and merged into master, then develop, branches or it should be merged into develop only and wait for release with SOCI 4.0.0.