load of misaligned address in libavcodec/startcode.c

[Suhwan]

Summary of the bug:
There're two UBSAN errors, which are load of misaligned address for type 'const uint64_t' (aka 'const unsigned long'), which requires 8 byte alignment

libavcodec/startcode.c:41:17: runtime error: load of misaligned address 0x619000000a81 for type 'const uint64_t' (aka 'const unsigned long'), which requires 8 byte alignment
0x619000000a81: note: pointer points here
 00 80 2b  68 65 61 64 09 31 2e 31  3b 0a 61 63 63 65 73 73  3b 0a 73 79 6d 62 6f 6c  73 3b 0a 6c 6f
              ^ 
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior libavcodec/startcode.c:41:17 in 
libavcodec/startcode.c:42:22: runtime error: load of misaligned address 0x619000000a81 for type 'const uint64_t' (aka 'const unsigned long'), which requires 8 byte alignment
0x619000000a81: note: pointer points here
 00 80 2b  68 65 61 64 09 31 2e 31  3b 0a 61 63 63 65 73 73  3b 0a 73 79 6d 62 6f 6c  73 3b 0a 6c 6f
              ^ 
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior libavcodec/startcode.c:42:22 in 

How to reproduce:

% ./ffmpeg_g -t 2 -y -i samples/h264/station.1080p.h264 -loglevel 99 -target dvd -map 0 -disposition: v:75 wmv2 -vframes 77 -r 108 -ab 868k -b:v 251k output/tmp.webm_dash_manifest
ffmpeg version N-94887-ge55018ee11 (git master)
built on ... ubuntu 18.04 with clang-6 and UBSAN option.