mm/memblock.c: fix NULL dereference error

mm/memblock.c: fix NULL dereference error

It causes NULL dereference error and failure to get type_a->regions[0]
info if parameter type_b of __next_mem_range_rev() == NULL

Fix this by checking before dereferring and initializing idx_b to 0

The approach is tested by dumping all types of region via
__memblock_dump_all() and __next_mem_range_rev() fixed to UART
separately the result is okay after checking the logs.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: zijun_hu <[email protected]>
Tested-by: zijun_hu <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>