FTL: Prevent InitializerList overflow

Users so far use the deduction guide, but this should not compile:

    ftl::StaticVector<int, 1> vector = ftl::init::list<int>(0)(1);

Bug: 185536303
Test: m ftl_test
Change-Id: Icc1263ebdb9326948f0fc2f9c9f01f21b0b8302d