Wrong result: DOUBLE/FLOAT UNSIGNED index scan with negative string constant

For an indexed DOUBLE UNSIGNED or FLOAT UNSIGNED column, comparing with a negative string constant such as '-1' returns wrong results.

=== Repro ===

CREATE TABLE t (d DOUBLE UNSIGNED, KEY(d));
INSERT INTO t VALUES (NULL), (0), (0.5);

– Indexed scan returns 0, but the row does not satisfy d = '-1'
SELECT d FROM t WHERE d = '-1';
– Result: 0 <-- WRONG

– Without index, correct result
SELECT d FROM t IGNORE INDEX(d) WHERE d = '-1';
– Result: (empty)

– Row-by-row evaluation also correct
SELECT d, d = '-1' AS eq FROM t ORDER BY d IS NULL, d;
– Result: 0=0, 0.5=0, NULL=NULL

=== Root Cause ===

Field_num::get_mm_leaf() stores the comparison value into the field before building the range. For UNSIGNED FLOAT/DOUBLE, a negative value like '-1' is clipped to 0 during this store. The optimizer then builds an equality range for 0, returning rows that do not satisfy the predicate.

=== Version ===

10.11.14-MariaDB

=== Fix ===

A patch is ready — will submit as a GitHub PR referencing this MDEV.