misc: fix memory leak in ifstat.c

misc: fix memory leak in ifstat.c

A memory leak was detected by the static analyzer SVACE in the function
get_nlmsg_extended(). The issue occurred when parsing extended interface
statistics failed due to a missing nested attribute. In this case,
memory allocated for 'n->name' via strdup() was not freed before returning,
resulting in a leak.

The fix adds an explicit 'free(n->name)' call before freeing the containing
structure in the error path.

Reported-by: SVACE static analyzer
Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>
Signed-off-by: David Ahern <dsahern@kernel.org>