libfdisk: fix get_partition_unused_primary()

Was:
Mentioned function returns -1 if adding of primary partition is
impossible. Caller treats this value as size_t (res variable) and then
compares it for negative values, totally ignoring errors.

Becomes:
Now function takes address to variable and fills it with partition
number. Caller treats return value as int and use it appropriately.

[kzak@redhat.com: - don't mix return code and partno at all]

Signed-off-by: Boris Egorov <egorov@linux.com>
Signed-off-by: Karel Zak <kzak@redhat.com>