msg: Avoid returning a negative value for nlmsg_attrlen()
msg: Avoid returning a negative value for nlmsg_attrlen()

If a hdrlen was provided that was greather than the actual
message length, a negative attributes length would result.

This was typically happening for RTM_GETLINK requests where
we can get a away with a 4 bytes header on the request side
but the response would use a 16 bytes header.

This resulted in strange -8 bytes leftover debug messages.

Signed-off-by: Thomas Graf <17c2d8920621522552b392f72d8fdb4d1e866b44@suug.ch>

mshirley/libnl,houstar/libnl,tgraf/libnl,honsys/libnl,tklauser/libnl,honsys/libnl,houstar/libnl,HolyShitMan/libnl-1,claudelee/libnl,cfriedt/libnl,honsys/libnl,teto/libnl_old,mshirley/libnl,cfriedt/libnl,UnicronNL/libnl3,baloo/libnl,HolyShitMan/libnl-1,karlhiramoto/libnl,vyos/libnl3,cyclops8456/libnl3,dsahern/libnl,teto/libnl_old,houstar/libnl,vyos/libnl3,tklauser/libnl,UnicronNL/libnl3,teto/libnl_old,vyos/libnl3,dsahern/libnl,karlhiramoto/libnl,tklauser/libnl,karlhiramoto/libnl,baloo/libnl,thom311/libnl,baloo/libnl,vyos/libnl3,congwang/libnl,cyclops8456/libnl3,karlhiramoto/libnl,kolyshkin/libnl,tgraf/libnl,thom311/libnl,greearb/libnl-ct,kolyshkin/libnl,kolyshkin/libnl,baloo/libnl,dsahern/libnl,claudelee/libnl,HolyShitMan/libnl-1,tgraf/libnl,congwang/libnl,mshirley/libnl,UnicronNL/libnl3,claudelee/libnl,claudelee/libnl,houstar/libnl,cfriedt/libnl,teto/libnl_old,tgraf/libnl,congwang/libnl,thom311/libnl,mshirley/libnl,kolyshkin/libnl,cfriedt/libnl,honsys/libnl,tklauser/libnl,congwang/libnl,greearb/libnl-ct,cyclops8456/libnl3,HolyShitMan/libnl-1,greearb/libnl-ct,UnicronNL/libnl3,cyclops8456/libnl3,dsahern/libnl