LU-11385 odbclass: Handle gracefully if nsproxy is NULL

LU-11385 odbclass: Handle gracefully if nsproxy is NULL

Gracefully handle the case if current->nsproxy is NULL:
check for the condition and return an error, avoiding attempts
to dereference the pointer.

Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: Ia102d2bacdb0e54b0339985396447e6d25465c56
Reviewed-on: https://review.whamcloud.com/36802
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>