Starting with clang-16, MemorySanitizer appears to check that
uninitialized values not be passed by value nor returned.
Previously, it was allowed to copy uninitialized data in such cases.

get_foreign_key_info(): Remove a local variable that was passed
uninitialized to a function.

DsMrr_impl: Initialize key_buffer, because DsMrr_impl::dsmrr_init()
is reading it.

test_bind_result_ext1(): MYSQL_TYPE_LONG is 32 bits, hence we must
use a 32-bit type, such as int. sizeof(long) differs between
LP64 and LLP64 targets.