This subtle change fix problem with too small amount of allocated
memory to store DFU function pointers.

One needs to allocate extra space for sentinel NULL pointer in this array
of function pointers.

With the previous code, the NULL value overwrites malloc internal data
and afterwards free(f_dfu->function) crashes.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>