#5 Check return codes everywhere

[Markus Elfring]

Some checks for return codes are missing.

Examples:
Would you like to add more error handling for return values from "strdup" like in the function "dbi_conn_select_db" and from "calloc" in the function "_dbd_result_set_numfields"?
http://libdbi.cvs.sourceforge.net/viewvc/libdbi/libdbi/src/dbi_main.c?revision=1.96&view=markup
http://libdbi.cvs.sourceforge.net/viewvc/libdbi/libdbi/src/dbd_helper.c?revision=1.42&view=markup