FindMatlab: Fix logic error when minimum version is not found

Fix logic added by commit 7fdd5128b1 (FindMatlab: Fix version selection
if a version is given, 2021-07-02, v3.22.0-rc1~66^2).  Ensure that
`_list_index` is always initialized to -1, akin to `list(FIND)` not
finding a match.

Issue: #22377
1 file changed