applier: fix use after free

Applier thread uses lsregion to allocate the messages for tx thread. The
messages are freed upon return to the applier thread using a
corresponding lsr_id.

Due to a typo, one of the lsregion allocations was made with a postfix
increment of lsr_id instead of the prefix one. Essentially, part of a
new message was allocated with an old lsr_id, and might be freed early
by a return of a previous message.

Fix this.

Closes #8848

NO_DOC=bugfix
NO_TEST=covered by asan in #8901
NO_CHANGELOG=bugfix