negative pid on Win32 causes DEALLOCATE errs.

[Ali ISIK]

In dbdimp.c (as far as I can see) the name of the prepared
statement is created using

    sprintf( imp_sth->prepare_name,
        "dbdpg_%d_%d",
        imp_dbh->pid_number, imp_dbh->prepare_number);

The pid_number for a Win32 thread can be negative, which causes
a syntax error, because postgres does not like, for example,

    DEALLOCATE dbdpg_-3228_1

Could an abs fix this?

Cheers.

-- Ali