libsnapshot:snapuserd: Fix off by one error in merge path

We only need to iterate exceptions_per_area times during
merge. Additional iteration overhsoots the buffer and hence triggers
the assert.

BUG: 168311203
Test: Full OTA with VABC - Verified merge complete

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: If6e8dee08802ea70a8fd40e93ad63a47f39ce96e