Fix harmless warning about signed/unsigned comparison.

MSVC (and probably other compilers too) gave a warning about comparing size_t
argument of postgresql_vector_into_type_backend::resize() with the expression
that became integer after the change of 29cb6c7e, so make this expression
unsigned again.
Merge pull request #132 from vadz/pg-warning-fix

Fix harmless warning about signed/unsigned comparison.