mtd-utils: nandwrite: avoid NULL buffer pointers

mtd-utils: nandwrite: avoid NULL buffer pointers

Commit 07005d915d6a79dbdee14b0c4360df5058c3a98b made changes to the
buffer allocation in nandwrite and did not handle all affected code
areas properly. In particular, we were assigning:
	oob.ptr = noecc ? oobreadbuf : oobbuf;
However, since oobreadbuf and oobbuf are declared dynamically, they
are NULL at this point. If they aren't properly assigned later, we
unwittingly are passing a NULL pointer as oob buffer.

This assignment line is best moved after the buffer allocations and
pointer assignment.

Effects of this problem can be seen when writing oob data with the "-o"
flag and without the "-n" flag:
	$ ./nandwrite -o /dev/mtd0 img.bin
	Writing data to block 0 at offset 0x0
	ioctl(MEMWRITEOOB): Bad address
	Data was only partially written due to error
	: Bad address

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>