rdma_counter_init() may fail for a device. In such case while calculating
total sum, ignore NULL hstats.

This fixes below observed call trace.

BUG: kernel NULL pointer dereference, address: 00000000000000a0
PGD 8000001009b30067 P4D 8000001009b30067 PUD 10549c9067 PMD 0
Oops: 0000 [#1] SMP PTI
CPU: 55 PID: 20887 Comm: cat Kdump: loaded Not tainted 5.2.0-rc6-jdc+ #13
RIP: 0010:rdma_counter_get_hwstat_value+0xf2/0x150 [ib_core]
Call Trace:
 show_hw_stats+0x5e/0x130 [ib_core]
 dev_attr_show+0x15/0x50
 sysfs_kf_seq_show+0xc6/0x1a0
 seq_read+0x132/0x370
 vfs_read+0x89/0x140
 ksys_read+0x5c/0xd0
 do_syscall_64+0x5a/0x240
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: f34a55e4 ("RDMA/core: Get sum value of all counters when perform a sysfs stat read")
Link: https://lore.kernel.org/r/20190723065733.4899-10-leon@kernel.orgSigned-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>