powerpc/boot: Only free if realloc() succeeds

simple_realloc() frees the original buffer (ptr) even if the
reallocation failed.

Fix it to behave like standard realloc() and only free the original
buffer if the reallocation succeeded.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229115149.749264-1-mpe@ellerman.id.au