lib: event-log - Fix handling dropping parent prefixes beyond a drop at a higher...

lib: event-log - Fix handling dropping parent prefixes beyond a drop at a higher level.

Before, the drop in the higher level became invisible to the lower
hiererarchies. For example:

parent1, parent2, parent3, parent4(drop one) yielded
"parent1: parent2: parent4: " as a prefix (which is still OK), whereas
parent1: parent2: parent3: parent4(drop one): leaf(drop 3) yielded
"parent1: leaf: ", while the expected prefix would be:
"leaf: ".

This means that the drop of one prefix by parent4 is ignored. Instead it should
apply that drop and operate on the prefixes that are still visible at the lowel
level, so the number of prefixes dropped should be additive, which is what this
commit changes.