the case where there's an entry in the manufacturer's device list but
no corresponding installation section (and hence no AddReg assignments),
i.e. if dev = find_assign(sname, "AddReg"); returns NULL, then
don't try to dereference dev.
There is a fundamental problem with the handling of .INF files that
contain definitions for multiple devices: right now we dump all the
AddReg sections that we find, but don't distinguish what device they
belong to. This often results in duplicate keys.