avcodec/startcode: Avoid unaligned accesses

Up until now, ff_startcode_find_candidate_c() simply casts
an uint8_t* to uint64_t*/uint32_t* to read 64/32 bits at a time
in case HAVE_FAST_UNALIGNED is true. Yet this ignores the
alignment requirement of these types as well as effective type
rules of the C standard. This commit therefore replaces these
direct accesses with AV_RN64/32; this also improves
readability.

UBSan reported these unaligned accesses which happened in 233
FATE-tests involving H.264 and VC-1 (this has also been reported
in tickets #8138 and #8485); these tests are fixed by this commit.

The output of GCC with -O3 is unchanged for aarch64, loongarch,
ppc and x64 (as well as for arches like alpha for which
HAVE_FAST_UNALIGNED is never true in the first place).
There was only a slight difference for mips and arm.
I don't know about the speed impact of them.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/startcode.c

@@ -25,6 +25,7 @@
  * @author Michael Niedermayer <michaelni@gmx.at>
  */
 
+#include "libavutil/intreadwrite.h"
 #include "startcode.h"
 #include "config.h"
 
@@ -38,14 +39,14 @@ int ff_startcode_find_candidate_c(const uint8_t *buf, int size)
      */
 #if HAVE_FAST_64BIT
     while (i < size &&
-            !((~*(const uint64_t *)(buf + i) &
+            !((~AV_RN64(buf + i) &
-                    (*(const uint64_t *)(buf + i) - 0x0101010101010101ULL)) &
+                    (AV_RN64(buf + i) - 0x0101010101010101ULL)) &
                     0x8080808080808080ULL))
         i += 8;
 #else
     while (i < size &&
-            !((~*(const uint32_t *)(buf + i) &
+            !((~AV_RN32(buf + i) &
-                    (*(const uint32_t *)(buf + i) - 0x01010101U)) &
+                    (AV_RN32(buf + i) - 0x01010101U)) &
                     0x80808080U))
         i += 4;
 #endif