Fail explicitly on length overflow.

Instead of aborting when FileMap::create detects an overflow, detect the
overflow directly and fail the call.

Bug: 156997193

Test: Ran unit tests, including new unit test that aborted before.
Change-Id: Ie49975b8949fd12bbde14346ec9bbb774ef88a51
Merged-In: Ie49975b8949fd12bbde14346ec9bbb774ef88a51
(cherry picked from commit 68604b9c)
(cherry picked from commit f846413e)