Edit overwrites wrong entry if label matches substring

Consider a scenario where you differentiate two papers from the same author in the same year be appending an additional character like so:

Mueller2020a
Mueller2020b

If you were to cobib edit Mueller2020b the changes will be written to both, Mueller2020a and Mueller2020b, causing the loss of the Mueller2020a entry and the duplication of Mueller2020b!

The reason is likely that the position check where the entry should assert that the label matches completely (i.e. it always uses the full length up to : and cannot match on substrings).