Coverity fixes

src/backtrace.cc

src/box/alter.cc

src/box/func.c

src/box/vy_cache.c

src/box/vy_index.c

src/box/vy_point_iterator.c

src/box/xlog.c

src/box/xrow.c

src/lib/msgpuck

test/unit/reflection_cxx.cc

test/unit/vy_iterators_helper.c

test/unit/xrow.cc