Fix an assert in KBK endgame

The endgame king + minor vs king is erroneusly
detected as king + minor vs king + minor

Here the fix is to detect king + minor earlier,
in particular to add these trivial cases to
endgame evaluation functions.

Spotted by Reuven Peleg

bench: 4727133
Annotate an unlikely condition

No functional change.
Merge remote-tracking branch 'upstream/master'