Fix get_quick_keys(): When building range tree from a condition
in form

  keypart1=const AND (keypart2 < 0 OR keypart2>=0)

the SEL_ARG for keypart2 represents an interval (-inf, +inf).
However, the logic that sets UNIQUE_RANGE flag fails to recognize
this, and sets UNIQUE_RANGE flag if (keypart1, keypart2) covered
a unique key.
As a result, range access executor assumes the interval can have
at most one row and only reads the first row from it.