iwlwifi: mvm: properly check for transport data in dump

iwlwifi: mvm: properly check for transport data in dump

BugLink: http://bugs.launchpad.net/bugs/1691369
commit c2e27e16f2411155b906db201b7e478144034ffe upstream.

When copying from vmalloc'ed memory to the SG list, don't crash
if the transport didn't provide any data.

Fixes: 7e62a699aafb ("iwlwifi: mvm: use dev_coredumpsg()")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>