powerpc/boot: Handle allocation failure in simple_realloc()

simple_malloc() will return NULL when there is not enough memory left.
Check pointer 'new' before using it to copy the old data.

Signed-off-by: Li zeming <zeming@nfschina.com>
[mpe: Reword subject, use change log from Christophe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20221219021816.3012-1-zeming@nfschina.com