settings: Fix check for duplicate when new value is smaller

The code for checking duplicates in the existing settings store was
incorrectly identifying a new value with the same initial content but
shorter length as a duplicate. Fix this by doing an early check on a
new vs old length mismatch, and immediately flag this as not a
duplicate.

Fixes #14840

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
1 file changed