zstd: Require at least 8 byte buffer in entropy_common

zstd: Require at least 8 byte buffer in entropy_common

This fixes the build on s390x which was rightfully complaining that
iend - 7 = buffer + 4 - 7 = buffer -3 is outside the array bounds.

../../grub-core/lib/zstd/entropy_common.c: In function ‘FSE_readNCount’:
../../grub-core/lib/zstd/entropy_common.c:121:28: error: array subscript -3 is outside array bounds of ‘char[4]’ [-Werror=array-bounds]
  121 |             if ((ip <= iend-7) || (ip + (bitCount>>3) <= iend-4)) {
      |                        ~~~~^~
../../grub-core/lib/zstd/entropy_common.c:77:14: note: while referencing ‘buffer’
   77 |         char buffer[4];
      |              ^~~~~~
../../grub-core/lib/zstd/entropy_common.c:105:30: error: array subscript -1 is outside array bounds of ‘char[4]’ [-Werror=array-bounds]
  105 |                 if (ip < iend-5) {
      |                          ~~~~^~
../../grub-core/lib/zstd/entropy_common.c:77:14: note: while referencing ‘buffer’
   77 |         char buffer[4];
      |              ^~~~~~
../../grub-core/lib/zstd/entropy_common.c:150:28: error: array subscript -3 is outside array bounds of ‘char[4]’ [-Werror=array-bounds]
  150 |             if ((ip <= iend-7) || (ip + (bitCount>>3) <= iend-4)) {
      |                        ~~~~^~
../../grub-core/lib/zstd/entropy_common.c:77:14: note: while referencing ‘buffer’
   77 |         char buffer[4];
      |              ^~~~~~

This is fixed in more recent zstd versions in basically the same way,
but the new versions needs more work to import.

Patch-Name: zstd-require-8-byte-buffer.patch

Gbp-Pq: Name zstd-require-8-byte-buffer.patch