FileMap::create: remove duplicate addition.

The previous change was intended to detect the overflow, but
accidentally retained the existing addition, so we'd abort before
getting to the explicit check.

Also reformat slightly to better match the current code in qt-dev and
beyond, to reduce merge conflicts.

Bug: 156997193
Test: treehugger
Change-Id: I73a3a4e75f0aad00888e8f2b49a07b7e8b4eda05
Merged-In: I73a3a4e75f0aad00888e8f2b49a07b7e8b4eda05
(cherry picked from commit 54794ac6)