vnc: fix unalignment access in tight_pack24

When adding '-fsanitize=undefined' in compiling configuration
and connect VM with vnc, it reports following error:

ui/vnc-enc-tight.c:910:13: runtime error: load of
misaligned address 0x621000466513 for type 'uint32_t',
which requires 4 byte alignment

This patch fix this issue.

Signed-off-by: Li Qiang <liq3ea@163.com>
Message-id: 20190318010442.14897-1-liq3ea@163.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/vnc-enc-tight.c

@@ -886,11 +886,11 @@ static int tight_compress_data(VncState *vs, int stream_id, size_t bytes,
  */
 static void tight_pack24(VncState *vs, uint8_t *buf, size_t count, size_t *ret)
 {
-    uint32_t *buf32;
+    uint8_t *buf8;
     uint32_t pix;
     int rshift, gshift, bshift;
 
-    buf32 = (uint32_t *)buf;
+    buf8 = buf;
 
     if (1 /* FIXME */) {
         rshift = vs->client_pf.rshift;
@@ -907,10 +907,11 @@ static void tight_pack24(VncState *vs, uint8_t *buf, size_t count, size_t *ret)
     }
 
     while (count--) {
-        pix = *buf32++;
+        pix = ldl_he_p(buf8);
         *buf++ = (char)(pix >> rshift);
         *buf++ = (char)(pix >> gshift);
         *buf++ = (char)(pix >> bshift);
+        buf8 += 4;
     }
 }