Several macros such as sint2korr() and uint4korr() are using the
arithmetic + operator while a bitwise or operator would suffice.
GCC 5 and clang 5 and later can detect patterns consisting of
bitwise or and shifts by multiples of 8 bits, such as those used
in the InnoDB function mach_read_from_4(). They actually translate
that verbose low-level code into high-level machine language
(i486 bswap instruction or fused into the Haswell movbe instruction).
We should do the same for MariaDB Server code that is outside InnoDB.
Note: The Microsoft C compiler is lacking this optimization.
There, we might consider using _byteswap_ushort(), _byteswap_ulong(),
_byteswap_uint64(). But, those would lead to unaligned reads, which are
bad for reasons stated in MDEV-20277. Besides, outside InnoDB,
most data is already being stored in the native little-endian format
of that compiler.
The crash is caused by macro uint3korr() accessing memory (1 byte) past
the end of allocated page. The macro is written such it reads 4 bytes
instead of 3 and discards the value of the last byte.
However, it is not always guaranteed that all uint3korr accesses will be
valid (i.e that the caller allocates an extra byte after the value).
In particular, the tree in Item_func_group_concat does not account for
any extra bytes that it would need for comparison of keys in some cases
(Field_newdate::cmp, Field_medium::cmp)
The fix change uint3korr so it does not access extra bytes.
