hw/net/rocker_of_dpa: Check group ID pointers are not NULL

In of_dpa_cmd_add_l2_flood(), we use rocker_tlv_parse_nested()
to fill in a tlvs[] array. If the guest command is valid then
the entries should be pointers to TLV data items with group IDs.
However, if the guest gives us bogus data then rocker_tlv_parse_nested()
indicates this by leaving the tlvs[] entries NULL. In the other
places that use this function, we check for this before using
the value, but here we forgot, and the result is that QEMU can
crash:

#0  __memcpy_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:331
#1  0x00005555574f7137 in __asan_memcpy ()
#2  0x0000555558106792 in ldl_he_p (ptr=0x8) at /home/pm215/qemu/include/qemu/bswap.h:278
#3  0x0000555558106755 in ldl_le_p (ptr=0x8) at /home/pm215/qemu/include/qemu/bswap.h:311
#4  0x00005555580f85ed in rocker_tlv_get_le32 (tlv=0x0) at ../../hw/net/rocker/rocker_tlv.h:114
#5  0x000055555810a8ad in of_dpa_cmd_add_l2_flood (of_dpa=0x506000082e38, group=0x503000b4e440, group_tlvs=0x7fff68702c20)
    at ../../hw/net/rocker/rocker_of_dpa.c:2032
#6  0x0000555558108a74 in of_dpa_cmd_group_do (of_dpa=0x506000082e38, group_id=1073741824, group=0x503000b4e440, group_tlvs=0x7fff68702c20)
    at ../../hw/net/rocker/rocker_of_dpa.c:2115
#7  0x0000555558108730 in of_dpa_cmd_group_add (of_dpa=0x506000082e38, group_id=1073741824, group_tlvs=0x7fff68702c20)
    at ../../hw/net/rocker/rocker_of_dpa.c:2135
#8  0x00005555580f66ec in of_dpa_group_cmd
    (of_dpa=0x506000082e38, info=0x514000072e40, buf=0x5070002356c0 "\001", cmd=7, group_tlvs=0x7fff68702c20)
    at ../../hw/net/rocker/rocker_of_dpa.c:2194

Check for NULL values and return an error.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/1851
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>