depmod: fix asserting mod->kmod == NULL

If we are replacing a lower priority module (due to its location), we
already created a kmod_module, but didn't open the file for reading its
symbols. This means mod->kmod won't be NULL, and this is just ok. Since
all the functions freeing stuff below the previous assert already takes
NULL into consideration, it's safe to just unref mod->kmod and let the
right thing happens.