boot: ReallocatePool() supports NULL pointers as first argument

Just like userspace realloc() the EFIlib ReallocatePool() function is
happy to use a NULL pointer as input, in which case it is equivalent to
AllocatePool(). See:

https://github.com/vathpela/gnu-efi/blob/269ef9dbc77ebec2723e0e6ae082bbca9516f5f1/lib/misc.c#L57