rrdtool.c: Improve error handling during database creation

rrdtool.c: Improve error handling during database creation

The case of filename pointer being NULL was not handled. The logic for
testing whether the database file existed at all should not return
true in case the file name is zero. As this function can't return
failure code, the error of NULL filename pointer is handled later when
the database is being created.

As many missing databases are created as possible. In case of errors
we will continue hoping that we will be able to update at least
something.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>