migration/xbzrle: fix out-of-bounds write with axv512

xbzrle_encode_buffer_avx512() checks for overflows too scarcely in its
outer loop, causing out-of-bounds writes:

$ ../configure --target-list=aarch64-softmmu --enable-sanitizers --enable-avx512bw
$ make tests/unit/test-xbzrle && ./tests/unit/test-xbzrle

==5518==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x62100000b100 at pc 0x561109a7714d bp 0x7ffed712a440 sp 0x7ffed712a430
WRITE of size 1 at 0x62100000b100 thread T0
    #0 0x561109a7714c in uleb128_encode_small ../util/cutils.c:831
    #1 0x561109b67f6a in xbzrle_encode_buffer_avx512 ../migration/xbzrle.c:275
    qemu#2 0x5611099a7428 in test_encode_decode_overflow ../tests/unit/test-xbzrle.c:153
    qemu#3 0x7fb2fb65a58d  (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7a58d)
    qemu#4 0x7fb2fb65a333  (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7a333)
    qemu#5 0x7fb2fb65aa79 in g_test_run_suite (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7aa79)
    qemu#6 0x7fb2fb65aa94 in g_test_run (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x7aa94)
    qemu#7 0x5611099a3a23 in main ../tests/unit/test-xbzrle.c:218
    qemu#8 0x7fb2fa78c082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
    qemu#9 0x5611099a608d in _start (/qemu/build/tests/unit/test-xbzrle+0x28408d)

0x62100000b100 is located 0 bytes to the right of 4096-byte region [0x62100000a100,0x62100000b100)
allocated by thread T0 here:
    #0 0x7fb2fb823a06 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:153
    #1 0x7fb2fb637ef0 in g_malloc0 (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x57ef0)

Fix that by performing the overflow check in the inner loop, instead.

Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

Author: quic-mathbern

migration/xbzrle.c

@@ -197,10 +197,6 @@ int xbzrle_encode_buffer_avx512(uint8_t *old_buf, uint8_t *new_buf, int slen,
     __m512i r = _mm512_set1_epi32(0);
 
     while (count512s) {
-        if (d + 2 > dlen) {
-            return -1;
-        }
-
         int bytes_to_check = 64;
         uint64_t mask = 0xffffffffffffffff;
         if (count512s == 1) {
@@ -216,6 +212,9 @@ int xbzrle_encode_buffer_avx512(uint8_t *old_buf, uint8_t *new_buf, int slen,
 
         bool is_same = (comp & 0x1);
         while (bytes_to_check) {
+            if (d + 2 > dlen) {
+                return -1;
+            }
             if (is_same) {
                 if (nzrun_len) {
                     d += uleb128_encode_small(dst + d, nzrun_len);