mm/memory_hotplug.c: add NULL check to avoid potential NULL pointer dereference

The NULL check at line 1226: if (!pgdat), implies that pointer pgdat
might be NULL.

rollback_node_hotadd() dereferences this pointer.  Add NULL check to
avoid a potential NULL pointer dereference.

Addresses-Coverity-ID: 1369133
Link: http://lkml.kernel.org/r/20170530212436.GA6195@embeddedgus


Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>